I'm sure it's got mostly to do with the fact that you can dispute credit card charges. If they were accepting credit cards and selling ONLY legitimate merchandise (which they try to do) it wouldn't be a problem. The problem comes up when somebody isn't happy with their purchase and they can dispute it with the credit card company.

I've disputed many Paypal charges (because I got scammed) through my credit cards and got my money back from my CC every time. Paypal will find in your favor, but if there aren't any funds in the person's account, they can't do anything for you. That's why I ONLY pay w/credit card when going through Paypal. I find that the CC company will most often side w/the buyer rather than the seller. I'm not sure who's pocket it comes out of in the end, but I've never had a dispute denied through a CC.

I'm sure these auction houses don't want to be bothered with people disputing charges (even if the item is a legit item).

Just my .02

-Brian