I cannot talk for other places, but in the context of the EU the big point is presenting yourself as a marketplace / a third party that merely stores shelf placement (or the equivalent on your platform), instead of selling others' items yourself.

Retail stores are not in the business of offering a marketplace for companies to buy shelf placement. When I go to a retail store and buy something, the seller is always the store. In terms of the laws, insurance, etc ... It changes a lot.

Marketplaces have protections, but those protection come from their independance from one vendor or another. Same principle that made us force all of our national ISP to separate their fiber/network business from their actual ISP business, or why the national train companies were cut and separated from the rail lines companies ...