Basically it goes like this for me. If I see anyone buying, selling, or passing loot that has been rolled on they go instantly onto my do not group list (which generally lasts a few weeks and forever if someone gets on it a few times).
If I see people buying/selling loot before a roll (not nearly as big a deal) I make a mental note of it. If I see them do it more then once I make sure that they never get any loot I pull. If they do it habitually I avoid running with on any quest where the loot is important. Frankly any buying of selling of bound loot is poor form if you roll on other people's loot yourself anytime.
Oh and if you ever trade for something under any circumstances and get your side of the trade and don't deliver your goods you should be perma banned from the game.