That leaves you vulnerable to fraud. The buyer says he didn't like the item or it wasn't as described and he sent it back. He keeps your bitcoins.
Thought of that already, thus the "real" listings would have to take place on established communities I think. This idea only works when there is already an established base of trust with the other users. In theory, any forum could be used to test these things, as long as they trust the users they are buying from (who are of course the ones putting up the eBay listings).
Fraud would be the risk in this situation.
The alternative is a system where we cannot sell Bitcoin through eBay at all, I don't like that system lol