Your question really has no bearing on whether the company in question is Ebay or Ford or the corner grocery store. Parties to a contract are bound by the terms of said contract. When one of the parties fails to adhere to the terms set forth in the document, the other party can choose to void the contract. In fact, when one party doesn't follow the terms, this voids the contractual agreement. One of the problems with businesses like Ebay is that because they want you to participate in their business model they commonly put vendors on "suspension" or probation. In the real world, you could file a civil suit and win if you adhered to the agreement and the other party did not.
If the dollar amount of your particular dispute is large enough, you might consider hiring a business attorney to help you with the matter. Otherwise, cut your losses and run.