Actually, eBay's TOS will make both of your arguments to be about contract law. The TOS is your contract with eBay.
After that I would say auctions are covered by auction law and buy it now is e-commerce.
This post is provided for general informational purposes only and is not intended to be legal advice specific to you. This general information is not a substitute for the advice of an attorney in your jurisdiction. The attorney client relationship is not established by this post.