It seems like once a week or so I have a buyer that has a problem paying through paypal... My answer is always the same and it seems to work pretty well. If you are getting a message that your payment cannot go through right now, try again later, then try these tricks:
1) Click on the tab that says, "Send Money". Enter the Seller's email address. Then check the box marked "For an Ebay Item". (This will usually work.)
2) I am not sure why, but I have had a couple of buyers from Spain that had to enter the amount with a comma ($1,00) instead of a period ($1.00). It worked and was the only way paypal would allow it to go through.
If all else fails, contact the seller. Ask them if they will accept a check or money order. It will take longer, but at least you will be able to pay for and receive the item you want. Also, be sure to contact the seller and let them know what is going on. Good communication can mean the difference in having a good or bad experience on ebay with each transaction.