I find it infinitely fascinating when games allow you to get away with whatever the hell is possible.
Scams, thievery, downright backstabbing. Some online games will explicitly forbid such practices. I find that weird. If it’s possible within the confines of the game’s engine, why would we not be allowed to do it? Hell, if you give people complex ways to interact and don’t force them into certain patterns, you instantly have a new layer of depth you didn’t have to write a single line of code for: mind games.
Let your cunning thieves be cunning thieves, let your con men be con men, and the heroes of your world will just have to learn to deal with them. Eve Online is a well-known example of this. Scams are completely legal, but so is infiltrating a corporation, rising up their ranks, and disbanding it after taking all their riches with you. As a result, trust isn’t cheap.
Just make sure to make in an explicit part of your game, and push its culture towards awareness of these practices. The last thing you want is people quitting because they never get to see justice after they got scammed.
Behind the screen, humans can be foul and wretched. Use that.