Maybe tell them they have to be express posted so that you can comply with eBay's policies on shipping the item within your handling time, as otherwise eBay will issue you with late shipment strikes.
Or, tell them the best way to avoid paying for postage costs is to purchase items that don't need postage. 
Seriously though, if you were willing to do it, and it's a parcel, you can get a free tracking sticker from the post office (I'd take a photo of the fully addressed package with the tracking label on it, to ensure you can provide proof of shipping to the buyer-supplied address), but you'd want to be 100% certain whatever stamps they send aren't going to raise any eyebrows at the PO. I've had buyers put clearly used stamps on return packages to me and it honestly really annoys me - one buyer just cut them off old envelopes, so kept the paper backing on them, and glued them on to the return package (and these were old stamps, a mix of 45c and others) - AP just put it in my PO box, no question, though. 
