A marketplace is usually associated with Allegro, Amazon, or a platform built from scratch for multiple independent sellers.
But there is another model. A marketplace can also be added to an existing online store.
The company continues selling its own range, uses its own warehouse, and fulfills its own orders. At the same time, it opens the catalog to external partners who can sell their own goods under the same platform.
This allows the store to expand its offer without having to purchase every product into its own inventory.
This is an area where agencies use a ton of corporate jargon, but it can be explained in simpler terms, which is what I aim to do in this article.
A Marketplace Alongside Your Own Store
Imagine a sports store selling running shoes, clothing, bikes, and fitness equipment.
Goods are purchased in advance from manufacturers or distributors, stored in a warehouse, and after an order is placed the store handles shipping itself.
As the catalog grows, more and more capital gets tied up in stock. Entering a new category means more supplier orders, more products to store, and the risk that some items will sell slower than expected.
This is where a marketplace layer can be added to the store.
Some products still come from the store's own warehouse, while others can be offered by external partners. The partner maintains their own stock and fulfills the order, and the product sits in the catalog alongside the store's own range.
The customer uses the same site, search, categories, and checkout flow. On the product page they only see who the seller is and who handles shipping.
If you have used Decathlon.pl, you have probably already seen this model. Some products are sold by Decathlon itself, others are offered by external marketplace sellers.
One Product, Multiple Offers
The same product variant can be available from multiple sources.
Say the store sells a specific running shoe model. Size 44 has just sold out in the store's own warehouse, but a partner has that variant in stock.
The product does not have to disappear from sale. The system can display the partner's offer and route the order to them for fulfillment.
The product page can be a single one, with only the offers assigned to it by individual sellers changing. The same product can be offered simultaneously by the store and several partners. Each offer has its own price, availability, fulfillment time, and delivery terms.
The system can automatically decide which offer will be the default. The store's own warehouse can take priority when the product is available, and once the store's stock runs out, a partner can take over the sale. The platform can also apply a Buy Box and automatically select the offer shown as the primary one.
New Categories Without Fully Stocking the Warehouse
That same sports store might want to expand into trekking.
With a classic model, you have to select products, order them from suppliers in advance, and stock the warehouse. Only then can you see how much real customer interest there is.
Companies that already carry the right range and have their own logistics can be invited to the platform. Their products appear in the catalog, partners maintain inventory and fulfill their portion of orders, and the platform earns a commission or another agreed settlement model. This allows you to gauge interest in a new category before the company decides to invest in its own stock.
The degree of freedom sellers have depends on the rules set by the operator. The operator can decide who is admitted, which brands and categories may be sold, and whether new products require prior approval.
A central product catalog can also be created, allowing partners only to add their own offers to existing listings. In that case the product page stays under the platform's control, and the seller manages mainly price, availability, and fulfillment.
The Customer Should Not See the Complexity of the Whole System
From the customer's perspective, such a store should work like a regular e-commerce site.
On a product page, what matters is price, availability, estimated delivery time, and seller information. There is no need to show the customer which system the stock comes from or how many warehouse locations the partner has.
The customer might see an offer for 449 PLN with 24-hour shipping from a specific seller, and below it the other available offers. All logic related to warehouse selection, reservations, and stock source stays on the system side.
Diagram of a single product available in multiple offers, from the store and external marketplace sellers.
One Cart, Multiple Sellers
The cart can contain products belonging to several different sellers.
A customer can simultaneously buy shoes from the store's warehouse, a watch from one partner, and accessories from another. From their perspective it can still be a single checkout process.
On the backend, the order has to be split between sellers. Each one should receive only their portion of the order and the data needed to fulfill it. The system must track all parts centrally — their shipments, payments, commissions, and any returns.
If the customer returns only the watch, the return concerns a specific seller and a specific part of the order. The other products may already be delivered or still in transit.
These cases need to be accounted for in the architecture from the start. Simply assigning a seller to a product is not enough to run a marketplace.
Inventory in a Marketplace
In a regular store, stock levels are relatively straightforward. A product or variant has a set quantity available in the warehouse.
In a marketplace, the same product can have several offers, and each of them can draw from a different inventory source. One partner might run their own Shopify store, another WooCommerce, a third might use an ERP, and a smaller seller might manage stock directly in the marketplace panel.
Every offer should have one primary source of stock level. If, for a given seller, that source is Shopify, the system synchronizes this value instead of maintaining a second independent state.
If a seller sells a product in their own Shopify store, the platform must also receive that information. Otherwise it may try to sell a unit the partner no longer has.
Webhooks deliver changes quickly, and periodic synchronization catches discrepancies caused by an API error or an undelivered event.
How to Keep the Storefront Fast
The storefront does not need the full warehouse and reservation structure. Displaying a product requires only information about availability, price, seller, and shipping time. The storefront can use pre-prepared availability data. When an order is placed, the system checks the current stock again and creates a reservation. This means two people can see the last unit of a product, but only one will be able to actually buy it.
Inventory from the Seller Side
For larger sellers, manually editing every product quickly stops making sense.
A company with several thousand SKUs needs search by SKU, bulk stock editing, import, and synchronization with their existing system.
If inventory is pulled from Shopify or an ERP, the panel serves primarily as a synchronization control dashboard. The seller should be able to see the current state, synchronization errors, and the time of the last update. A smaller seller can still manage stock manually. Both cases can work within the same platform.
You Decide Who Can Sell
Running your own marketplace gives the operator control over who can sell and which products appear in the catalog.
Decathlon selects its marketplace partners and sets requirements around order fulfillment and customer service.
Similar rules can be applied in your own store. Some businesses can start with a handful of existing suppliers and gradually expand the number. New products may require moderation, and selected brands or categories may be available only to specific partners.
Many marketplace platforms operate as a closed network of vetted partners. For an existing store, this is often a more natural model than open seller registration.
Who Has to Ship the Goods?
Zalando combines several collaboration models with brands. In classic wholesale it buys products from brands and manages their onward sale. In the Partner Program, products can be sold by partners directly through the platform, using their own logistics or Zalando's fulfillment infrastructure.
A similar model can be applied in a smaller e-commerce business. Some orders are fulfilled from the store's own warehouse, some by sellers. The system simply needs to know who sells the product, whose stock it is, and who is responsible for shipping it.
Who Is This Model Right For?
Extending a store with a marketplace is particularly interesting for businesses that already have customers, traffic, and supplier relationships, but want to grow the catalog without keeping all the stock in their own warehouse.
This model is easy to picture in sport, electronics, car parts, or specialist B2B. The common denominator is a wide catalog, many suppliers, and significant capital required to maintain full stock.
For a small D2C store selling a dozen of its own products, the extra layer may not bring enough benefit. It adds seller management, order splitting, stock synchronization, billing, and returns — so the model should address a real business problem.
You Do Not Have to Build a Marketplace from Scratch
An existing store can keep its own catalog, stock, and current sales model, and launch external partner offers alongside it, integrated into the store.
At AppCrates I build marketplace platforms on Medusa.js, both as standalone multi-vendor systems and as extensions of existing e-commerce stores. When extending an existing store, most of the work typically involves the offer catalog, inventory, order splitting, fulfillment, and synchronization with seller systems. The architecture must be tailored to the way the company actually sells and fulfills orders.
This solution makes a lot of sense for growing brands, but not only them. You do not need to hit magic sales figures. All it takes is the desire to enter a new branch of commerce, without investing in goods and their handling. That is why, despite appearances, this is a solution for both large players and smaller stores in global as well as local markets.