Most sellers are not holders of merchant accounts with a credit card company. Instead we use Paypal .
Are you registered with Paypal yet? It is free for the buyer and has protections for buyers.
Are you trying to use a prepaid credit card? Most of those are not accepted by Paypal because they have limited funds and even more limited identification of the holder.
You need a 'real ' credit card backing a Paypal account for the simplest and most protected buying on eBay.