Just a suggestion, but what about having the person getting the goods pay the postage via paypal in advance. That way you know you will not be out the postage if they don't send something back. I know it seems redundant to send them postage and them send postage back, but you would see who is really going to follow through. That what if you have an issue as you said with too many requests the postage would be prepaid.
Huh? What did I miss?