Master Card is an accepted payment method for eBay so there may be one of several possibilities.
1. Make sure the CC information is correct in eBay. I have had this happen a couple of times when my card expired and they send me a new one with a new expiration date and I had not updated it in eBay.
2. EBay may have not accepted the card because the CC company refused the charge for some reason. Some CC companies will refuse a charge if they detect what they consider to be a suspicious charge that is not consistent with your normal charge card habits. You may want to contact your CC company if everything is accurate in #1.
