The walking, talking ATM and his light fingers are actually two scams rolled into one—or, technically, a scam wrapped in a pickpocketing.
A stranger offers to help you change money. Alarm bells should go off immediately.
While in some countries, buying currency on the gray market like this can work in your favor (but you have to know what you're doing), those countries are no longer found in the developed world.
No dude on the street can give you a honest rate that's better than what you'd get at a bank. Besides, this is one major way local criminals dispose of counterfeit bills. Just say no.
OK, say you skipped that last paragraph and fell for the scam anyway—we'll even give you a lame excuse for it (it's 3am, you just got off the train in a new country, you have nothing but some emergency dollars, everything is closed, the ATM in the station isn't working, and the only hotel still open in town demands cash up front. Congratulations: you have just repeated my first night in Krakow in 1994).
So you dabble with the dude and his gray-market money.
He is a warm and friendly guy, your money-changing savior, and after the deal is concluded, he goes all Mediterranean on you, exclaiming, "You are my new American friend!" And he wraps you in a big bear hug to say good-bye.
You stand there, bemused by his effusiveness, and he walks away with a wave and a smile.
Congratulations, he switched gears from con artist to pickpocket and just stole back the money he gave you, plus whatever else was in your wallet.
Now, not every gray-market money changer throws a wallet-lift onto the end of the shady transaction, but since you avoided step 1 to begin with (the scam) you luckily never have to find out which kind he is.