In the even the buyer has not paid (or just paid you), I would contact ebay. If the buyer is asking for their money back, and they have not paid for the items, it sounds like a scam. If they just paid and want their money back, refund them now and avoid the hassle of a return. Irregardless call ebay first, so you have them in your corner if the buyer tries to leave negative feedback.
If this is an issue that you wanted your payment to clear (and you think you can sort it out with the buyer), I would advise to be apologetic and explain that on your end it appeared payment was not received. If they want their money back, refund them.
