I'd still like to know why the buyer searched Gumtree for another seller then contacted him after the purchase... That's weird to me. It's not something people normally do. Where there is smoke there is fire. My guess is the OP suspected something was off.
Is it unusual?? News to me.... Personally i do it every time i buy something.... i want to confirm to myself i got a good deal or not by monitoring similar items for sale, Its called 'curiosity'.