Having bought and sold there the rules are quite arbitrary, and their cryptic algorhitm is a nuisance to buyers (you clicked by accident on a stove? You’re gonna see a ton of stoves in the recommended for a while!) and periodically harms sellers (if you don’t post daily and basically make it your day job, good luck making money!)
a federated alternative, with different instances for various interests and categories, meta-categories even and so on. Maybe regional instances like we have on here, one for the EU (quite convenient to ship and receive packages from inside of it, no customs wasting time and money) one for North America, one for East Asia, etc. With one being able to purchase from all of them.
Federation would also ensure that rules are properly enforced without abuses or other malpractices like eBay does (did you know eBay shipped a pig head to somebody who publicly criticized them?) since those instances would naturally be avoided and new ones would be made. It would also prevent excessive fees, as the fediverse is generally not a for-profit endeavor, and still, there will always be the option to shop around from other instances.