Q plate indicates that it was properly registered, a lot of the early cars ran on the donor logbooks, some may well still be. I put mine right at the amnesty before SVA came in. So a Q plate may actually be beneficial to a new owner. I believe that a few people have managed to get age related plates after proof of donor age?
Recent sales have been exported and the pound is at a low point at the moment so who knows? Gumtree seems an odd place to advertise it though. Maybe it'll appear on some of the more usual sites soon?
Guy