Because eBay is a private company, they are not really obliged to comply with principles of US jurisprudence, any more than Blogger, for example, is required to abide by the First Amendment.
They are providing a service "at will," as it were, and it is their will, and not yours. They own the servers, they pay for the bandwidth, the software, etc. and they have the option of providing that to the public either free of charge, or for whatever price they choose to set, and to cease doing that when they choose to do it.
The board's legal eagles I am sure will correct me if I err, but I believe that the only laws that eBay would be bound to where sellers are concerned might have to do with "seller fees."
For example, if they accepted your seller fees, and then refused to accept your item before the time you had paid for had elapsed, in order to keep the fees without providing you the service you had paid for, I think they might have to be able to demonstrate that the item violated their terms of service in some way, which I imagine they could do in pretty much any case, regardless of what the item was, since it would have been their lawyers that signed off on the terms of service, and the purpose of those terms is to serve the company's interest, not yours.