Phone calls go a long way.
I have a rule of thumb for buying/selling on gumtree, forums or FaceSook: if there's more than four messages without a phone call (or a phone number being offered), then they're probably a dickhead.
It's not 100%, but it is close.
And as a potential buyer, it's surprising how often people remark "you bothered to call!" - most sellers get way too many scammers and morons messaging them, so genuine potential buyers are missed if they don't call.