This has been suggested many times, I went through and looked, can't find many of them, guessing they've been deleted. but the reason that it always gets rejected is because of the possibility of scammers and whatnot. really, it's a "we don't want it because we don't want to get blamed for anything that could go wrong," and it's a valid reason.
really the only way i could see this being done in a manner that would protect the site and its owners from any responsibility would be doing it on a sub-domain or something, like shop.codinghs.com, with a completely different set of rules, and making people have a separate account and all sorts of stuff about "this site makes no guarantees about the validity of the things sold here, please make sure that you check the other person's reliability records and stuff"
and even then, i'm not sure that the possibility of trouble would be worth the hassle.