A marketplace aggregates the best possible products of an industry into one place.
To do so, they have to find and attract the best vendors (individuals and/or businesses) to bring rarer products on their platform.
Another type of e-commerce store is drop-shipping, which instead bring value by finding a new market (or a new branding) to a product.
Traditionally, an e-commerce store is an online store of a brand managed by the maker(s) of that product.
Marketplace are now trendy because it's an efficient solution for customers that are looking to shop local and second-hand products.