Magmarock: As a suggestion, you might want to make that more obvious on your website or something.
"Don't buy keys from anywhere else but here" or something.
Thing is: from time to time
(very seldom, admittedly) they do sell
LEGIT GOG keys on HumbleBundle, etc.
So, if they were to put out a warning like you suggest, people would probably refrain from buying on those rare occasions, thinking they don't receive legit keys for their money.
As a rule of thumb I would advise people to only buy from known legit key sellers
(GMG, GG, etc), and refrain from buying on sites that are by now known for (re-)selling illegitimately aquired keys.
One good indicator is usually also a very low price.
Remember:
if something sounds too good to be true, it probably is not true.
Very recently I read an article about concert goers being ripped off by Ebay ticket (re-)sellers.
Two tickets for a three days festival plus camping for €90,- each...when the real deal would have cost them €120,- each.
I mean, at some point, people have to realise, that nobody is (re-)selling for only three thirds of the price they themselves had to pay.
Mind you: we're not talking about some guy on the parking lot in front of the event, holding up a sign "Girlfriend left me yesterday - tickets to sell". There you may get a cheap one, if the guy is desperate enough.
But these guys bought weeks, if not months prior to the event, and probably grinned from ear to ear all the time because they made such a great deal.
Until they were refused entry on day one of the festival.