The angle that I would be looking at this from is whether eBay can ask its sellers to comply with the terms - if they can, then they can't get into trouble for enforcing them, because by using the site once the policies are in force, a member has agreed to comply. In other words, is it ok for them to make it so that if you sell on eBay (which is voluntary), you have to be prepared to provide (let's just call them) consumer guarantees above and beyond what you would normally be required to offer. 

I don't know the answer to this question, but I think it's the most important one to ask at this stage.