I've even had the same situation on Marktplaats (the second hand site I think you are talking about.) Sold a lens to a guy, we chatted a while, he said he was looking for a telelens and by chance I had that one. Wasn't really interested to sell but what the heck so I sold that also. Few days later he came by to say that he didn't like it and brought the whole lot back... Sh*t happens, I guess!