My personal view -- having spent all of a few seconds thinking about this, having only the most rudimentary background in antitrust law, and reflecting exclusively my own PERSONAL opinion -- is that Ebay does have significant market power and is engaging in an illegal tie by forcing sellers to use the PayPal product that they don't want to avail themselves of the ebay marketplace, which they do. I would further argue that, having at one time permitted alternative forms of payment, any subsequent reduction in the breadth of those options is violative of the essential facilities doctrine (which basically says you can't lure a consumer in by making it seem like they have options then divesting them of those options). But are you sure buyers can't send you a money order? And that PayPal is the only option Ebay permits? I'm not sure that's true. That it's the only option the consumer prefers isn't Ebay's fault.
Yes, I'm sure. They mask it in this "assurance" bullshit, but its a scam. CCards provide assurance, banks provide assurance. PayPal acts like its the only one.
To your overriding point: say you bought a windbreaker (I know, unlikely) from whocares.com. Then you get to payment screens and you see that they only take MasterCard. WhoCares is a bigtime online retailer. So they have the power to accept different forms of payment, but for some odd reason they don't.
You hire a lawyer. Smash cut to an ebay transaction: suddenly you are faced with 1 option, I need a paypal account, username, all the bullshit, and now they control the fees outside a competitive marketplace. Yes, you can use a CC, the best option since paypal and cc compainies have "deals".
So in instance 1, its MC, otherwise both. The point is that with every large scale business you have choice which fosters competition. Ebay/PayPal cartel do not...