Well, still doesn't matter if we don't know what is his gog name, he contacted you via steam therefore there's nothing to do here.
As for a chargeback, if you had bought the key somewhere you could ask the bank to refund you (by lying and saying that your cc was stolen or something like that), the store would revoke the key and perhaps put you on a black list (steam would ban you), dunno, it's a shitty situation because if i'm not mistaken the store also pays a fee for the chargeback so it wouldn't be fair for them.

I would say that the chance of gog support helping you is... 1%, it has to be something of their goodwill because officially there's nothing they can do since it was a transaction made by you with the guy, nothing to do with them...

Chargeback - When you make a purchase from a merchant with your card there is an agreement that you provide funds for a service or product. If the merchant does not provide the goods or service then you can contact your card issuer to file a complain. They will refund you, then contact the merchant's payment processor to be paid. (note that you get refunded before they do - this is one of the advantages of using a card). The payment processor will then want their money back so they will take the money from the merchant who was the one who got paid for service or goods not delivered.

Usually a chargeback is last resort, most disputes can be resolved between you and the merchant, but if the merchant goes bankrupt, or is commiting a fraud then chargeback is your solution.

Therefore after writing all of this, chargeback is definately not your solution. You could go to steam and declare the key stolen, if you have proofs of the discussion you had with that trader then maybe they can revoke it or give it back to you. At this point you should try anything...

Look here:
https://support.steampowered.com/kb_article.php?ref=9958-MJDG-3003#thirdpolicy

There seems to be a way to report these people so maybe you do have options.

^ It's a gog key and like i said, it's probably from that graphic cards promotion.

Some greedy people stop at nothing. These stories here really break a gamer's heart

First time I've seen this. Thanks for the mention. Talking shit and doing shit are two completely different things, but I do love the irony of my name being mentioned here in this context :D

seems that drepping with alt accounts is a very time-consuming task.

I'd say it wouldn't be if one did it the right way.

Aw Gragar's screenshot of my monumental Number of the Beast rep day is no longer accessible. As you can see from the attachment below and compare it to my current rep, some certainly do enjoy spending their time "drepping" :D

DanielX99, possible phisher, for he offers GoG games at half price with claims of legitimacy, yet there is a possibility that he/she will need a credit card.

^ was about to mention this - at any rate one to be wary of as they're randomly approaching users via chat (and now we dont have a "block" option, location or any other identifying details within the messages page - well done gog!

Looks totally legit.

The best part is the absolutely GLORIOUS grammar and spelling.

I can't even believe he's a real phisher. I mean, such retardation. Even scammers put in more effort than that, he might just be a troll.

Your avatar cracks me up :D

Daniel just hit me. I gave him j0ekerr's credit card.

Seriously, should I say anything like "Fuck off" or just ignore him?

Of course I could friend him and see what he does. I'd like to see his wishlist.

I'd answer something like "You make me good price. Suck good. Me like." oO

the best part comes after :)

i menaced to report him as potential scammer / phisher -> answer : "please no, i only selll"...... yes sure i trust him :P