DaasYochid if you let me into your house only as long as I do not make legal agreements with anyone and while I am there I enter into a legal agreement with someone does that make that legal agreement invalid? Only the law can affect the validity of contracts made between two parties, no matter what platform was used, legitimately or illegitimately.
Poor analogy. By listing on eBay, you have entered into a contract to follow their rules for this transaction, not for some side transaction off of the site.