I used woocommerce in the past. Its not that complicated. Woocommerce is open source from what I read: www.forbes.com/advisor/…/woocommerce-review/ i would have to check the source but implementing federation would be quite trivial i guess.

Why bother federating:

You advertise for your partners, not competitors. This is done already but manually by reselling. This would just expedite the process. The only part that is not yet clear to me is if the shop advertises something from another shop and clearly says, only sale processing through website, not fulfillment, if that would also make it that the legal warranty is done by the downstream vendor. Processing returns also is trivial from a technical perspective. Its just the legal one that keeps me guessing atm.