If it is over say $500 or a thousand, use escrow. For less expensive domains, check the reputation of the seller or ask for references.
Co-Founder@HostHideout. Profoundly influenced by #Bauhaus, @Nameslave unrepentantly embraces #Minimalism with a bias for functionality, color theory and pixel precision: a #multimedia messenger in the McLuhan sense. His totally irrelevant M.Ed. dissertation examines Organizational Culture and Change Management. He also likes Patrik Ervell, Wong Kar-wai and IKEA.
I would recommend using the search function for the forum you are using as a medium. Check out the person and see if anyone has complained. You could also possibly ask them to do a 50% deposit, transfer, and then you pay the balance. I have brokered over a hundred domains and I have never done that myself though.