If you are trying to purchase items from multiple sellers, see if you can purchase each separately. There should be a checkbox in your cart that says "pay only this seller" or something similar for each line item. Some sellers may require different payment methods, or may not be set up to allow their purchases to be combined, and these may have to be purchased separately.
Is your seller new to selling, or from outside the US? You will likely have to contact the seller to see if there is a problem on their end -- if they have problems accepting payment in general or just from you.
