The comissions on credit cards are mainly directed for big commerces, shops and companies that have comisions on the items they sell. The idea of a "service" that you give to the client is also true for big corporations and shops, but nothing of this applies to particular sellers like most of us here. We don't have a fixed commision when we sell items on ebay and many times we lose money.
Even if paypal is a service that costs money and someone has to pay for it , now that paypal has been integrated with ebay they should do a complete integration and waive the payment fees or consider them included in the final fee. As things are now it's not rare to find yourself paying more as paypal fees than ebay fees, and also that you are paying double fees..... But of course they will never do something like this... it's their money tree after all

as for when they contact you about an item that fails to comply their policy, well, it's not that they wait until the auction finishes or wait until someone tells them. Of course they can cancel your auction if someone tells them that you are surcharging, but they also have automated bots that scan the auctions analyzing posible faulty listings. Then it's just a matter of when/if the bot catches you or not. If you are caught by the bot someone will revise the auction and then cancel it. Of course this can apply to recently finished auctions but for sure they wouldn't do anything if the auction ended a few days ago as it would be too late.