I think it's likely that there is rep trading, where users rep each other just to gain higher rep .. trustworthy users usually have been at dp a long time, have good reputation, have good itrader and have a fairly high amount of posts.. all of which can be checked in a user profile.
The best thing you can do for anything if you have been taken advantage of, report it to as many places as possible. At least let the cheater know that others know.