I don't really know if this can implemented well but to help point out deceitful traders and scammers I was thinking that maybe a rating system could pop up after every trade (or even something as simple as a thumbs up/thumbs down). Average of the value could show up as a number next to the player's name in market worlds. If possible trades including rares would be worth more per vote to avoid people just trading common items to raise their ratings.
An incentive can also be given to those with good ratings (loot bonus in dailies?) idk