Battlenet Key works but scammed?

Officially, we don't encourage or condone the trade of CD keys. There's a lot of risk associated with it, and not a lot of accountability in where CD keys come from. This is especially true on websites like G2A.

When it comes to new AAA titles, the old adage "if it sounds too good to be true, it usually is" applies. Cheap games come from prior sales or sometimes bundles, which will not have happened with a brand new and expensive title. If someone offers a game below market price, it's usually because they bought it on a stolen credit card, and are trying to sell it quickly before the chargeback happens, leaving you stuck with the resulting mess. You won't have any problem redeeming the key, as you saw, but sooner or later it will probably be revoked, at which point you'll lose access to the game.

Charging back now would constitute fraud on your part, on the unlikely offchance the key is legit - he did give you a working key, after all. I would recommend you try contacting G2A, explaining the situation, and asking them to investigate or help you out. You may realistically not get a favorable answer from them, if what I've heard is true, in which case you should simply cut your losses and learn from the experience. We can't do anything to help you out here other than recommending you avoid trading for codes from here on out.