Oh no not again.
To legally play a game, you need a
license, a permission from the rightsholder. Playing a game without a license is copyright infringement.
A key is not a license and doesn't grant a license, no more than having a key to a house grants you the real estate title.
Selling keys is really, really questionable. Selling keys is basically the same as posting magnet hashes. Are magnet hashes copyright infringement or protected speech? Who the fuck knows!
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Aside from copyright infringement, key sellers can be also locked up or fined for fraud (telling customers they're buying licensed games) or unjust enrichment (profiting off something
you have no business profiting off).
Buying keys is questionable in exactly the opposite way. If key selling is deemed illegal, a buyer might be able to claim to be a victim of fraud. On the other hand, if key selling is deemed a bullshit but
legal service for "people who already have licenses", the buyers are infringers and have to face fines and possibly jail sentences.
...I miss SimonG so much.
Trilarion: cdkeys says:
"With an extensive, reliable supply chain and purchasing power, cdkeys.com can aggregate sales volume - we then pass these bulk reductions on to you, which ensures our pricing is the most competitive around!"
So it may simply be their superior purchasing power skills. The department where GOG sometimes fails (for example with regard to regional prices).
Quite unsurprisingly, this is bullshit flavored bullshit. "Purchasing power" doesn't apply for game keys at all, and legal resellers aren't free to undercut pricing by passing their commission onto customers at their leisure. The one market entity which can be said to have the capability to enforce game prices is Steam, and we know for a fact they don't generally do it, perhaps outside of edge cases ("your $999 match-three game looks like money laundering and bad PR for our company, so we won't be selling it").
edit
Regardless of the (negligible) risk of fines or jailtime,
buying keys on unauthorized websites doesn't make the rightsholder owe you a license. There isn't a requirement for rightsholders to spend resources on chasing down scammers or lose their IP.