How much money are we talking about? If it was only around the $10 plus postage mark I would send them the item with a note to tell them to have it on me to either gift or resell and that I had given them a full refund.

I(f it is a lot more money and they do not contact you then you have to decid how to proceed. You can just refund them which may get you a neg, you can send it with a note saying if they do not want it they can either have a partial refund and keep it or you will pay for rturn postage and refund them in full when you get it back. 

Quick edit, didn't take note of the time scale, you need to give them longer to reply.