BT Rep is a way to gauge a user's reputation amongst other traders. It's not 100% fool proof, but is still one of the key things to look at when trading with a user.
The idea behind it is that when you trade with another user and the trade goes through without a hitch you give them a rep point.
DO NOT give rep points for any other reason.
If a user messages you asking you to trade rep points, or to give them a rep point before going through with a trade, report them (and don't give them the point!):
Click Here
The way BT Rep works is explained in this thread:
Click Here