Let me give you a real life example:
If you put your card in an ATM and ask for a specific amount of money, unless the machine is broken or you don't have the funds, you will definitely get what you asked for. This in-game is what vendors do - if you have the credits or the comms or whatever, they give you exactly what you want.
If someone you don't know volunteers to get the money for you and you give him your card and your password, chances are he will empty your bank account and leave you wondering wth happened.
You seem to forget that behind these in-game pixels, are people. And that some of them are not as innocent as they seem.
What the guy did was not Nobel-prize smart, so much as devious. However, the fault is only yours for trusting someone you don't know.
I don't know about this game, but I know in WoW you can get banned for trade ganking. All you have to do is screenshot the conversation and send it to a GM.