Marketplace is the term used for websites which feature lots of independent businesses in one place. The website itself isn’t selling the product, the “vendors” are. In this case, the website acts as a platform or an agent, often processing transactions, payments and facilitating customer service.
Many independent retailers use marketplaces because of their superior marketing and technology – think of the masses of traffic that Amazon gets, or the usefulness of the postage options and tracking updates on eBay.
Marketplaces often take a commission or charge a monthly fee for the services they provide to the vendor and purchaser.
The EU requires marketplaces who handle payments to have a payments licence. There’s more on that here.