See if they offer you their cellphone number. Theive selling stolen bike usually don't like giving away telephone number. Ask specific question about the bike. I once met a seller in front of Mac Domald selling a Alvalanche mtb. He claim the bike is clean and he's been riding it everyday. The guy was over six feet tall and the bike was medium frame bike with the seat adjustment all the way down. That raised the red flag for me. I've seen better bike fitting on taller guys riding small bmx.