At what point is your payment being declined? What message are you seeing?
Just one particular seller?
Is your seller new to selling or located in another country? It is possible that the seller has not set up the account to receive your payment.
If the problem happens with multiple experienced sellers, that would indicate the problem is more likely at your end, or at eBay's end. Check your PayPal account to make sure you do not need to update anything. Try another payment method.
