The problem with that is that they can forward it to a collection agency, whom when told to screw off, replies by screwing with your credit score.
It was honestly far easier and satisfying to just give them the two bucks and then report the facts of my experience onto the internet.
Plus, with any luck someone thinking about selling on eBay in the future will run across this post, think twice, and be saved the trouble I was. So I feel pretty good about that too.
If you agree to use a service without reading the fine print, then you're obligated to what you agree to. "Nobody reads it" is not an excuse that a judge uses to nullify a contract.