From a seller's standpoint, Ebay has one significant issue.

Ebay pushes the use of Paypal really hard, to the point where many sellers won't accept any other form of payment. Ebay convinces sellers that Paypal is a safe, secure form of payment to accept, but they don't support sellers at all.

Basically, if you want free stuff from Ebay, all you have to do is pay through paypal with a credit card and then claim the purchase was fraudulent. 100% of the time, paypal will refund your money because they don't have the clout required to argue with major credit card companies, and they sure as hell aren't going to be stuck with the bill.

This complete lack of support for retailers is slowly killing the site.