Most of the time you have to pay an EXTRA premium to use a credit card. You have to look at the auction terms. Like with a 10% buyer's premium:
Say you buy $100 game. You have to pay an additional 10%. So now it's $110. Then you have to pay sales tax, and they usually calculate that on the $110. If it's a 7% sales tax, you then owe $117.70. Then you owe an additional fee of say, 3% for using the credit card. That's on the total amount. Now it's $121.23
So for every hundered dollars you spend, you have to pay an additonal $21.23.