Hi Felipe Ramirez,
Unfortunately ATG is not providing Out of the box support for Market place solution.
We may need to Integrate with various seller and get their catalog, pricing, inventory as a feed and place the order from ATG to Seller for fulfillment.
Using this way, we will end up multiple integration with each seller (minimum integrations for Catalog, Price, Inventory, Order submission, Shipping Information, Product return etc...)
Seller registration can be managed like, What we do for B2B Customer registration.