A game key for a fraction of the official price is tempting. But a key has to come from somewhere, and on the grey market that somewhere is often the source of the risk.
What the grey market is
The grey-market for game keys is third-party resellers and marketplaces selling activation keys outside a game's official store. Some keys are legitimate — bought cheaply in a region, from a bundle, or in bulk, then resold. Others are not. The trouble is that from the buyer's side, the legitimate and the illegitimate keys look identical until something goes wrong.
The fraud problem
The most serious risk is keys bought with stolen payment details. A reseller, or someone supplying them, uses stolen cards to buy keys cheaply and sells them on. When the fraud is discovered, the publisher can revoke those keys — and the game simply disappears from the library of whoever ended up with one. The buyer paid real money for a key that was never the seller's to sell.
Other risks
Even non-fraudulent grey-market keys carry risks. A key may be region-locked and refuse to activate where you are. It may be a different edition than advertised. There is usually no real support if something fails — the official store would help; an anonymous marketplace seller may not. And buying this way offers no recourse if the key is simply dead on arrival.
Why publishers dislike it
Publishers and developers see little of grey-market money and bear the fraud costs, which is why they revoke fraudulent keys and discourage the market. For the buyer, the practical point is not the ethics — it is that a revoked key is an unrecoverable loss.
The takeaway
A grey-market key might be a genuine bargain, or it might be fraudulent, region-locked, or dead — and you cannot tell which until you try it. The safest purchase is from the official store or an authorised seller. If you do use the grey market, treat the low price as carrying a real risk of buying nothing at all.