The biggest problem for a fully crypto ebay is that it will suffer from the opposite problem that ebay does. Ebay suffers from rouge buyers, who receive the item and then complain that they didn't. Ebay always sides with the buyer since there are more of them and reverses the charges.
Since there are no reversible charges in crypto, a crypto ebay will suffer from rouge sellers, who wont ship the item, but will take payment.
The only possible way of resolving this is to have a trusted physical distribution center which sellers post their items to, the center then confirms the item, forwards it on and acts as escrow for the payment as well.
This is fantastically difficult and expensive to do in practice.
What we need is way to get as close to this as possible with the least amount of pain.