Where the key came from
Two stores can sell the same game at prices that are nowhere near each other, and the reason is almost never that one of them found a better wholesaler. It is that they are selling different things. An official reseller buys keys from the publisher or its distributor and sells them to you directly. A marketplace does not hold stock at all: it hosts listings from third-party sellers, takes a cut, and the key you receive was bought by someone else, somewhere else, before it was sold to you.
Neither model is fraud, and the large majority of marketplace purchases go through without incident. But the second model has a failure mode the first does not. A key originally bought with a stolen card, or bought cheaply in a market where the game costs a fraction of what it costs where you live, can be revoked by the publisher after it has been activated on your account. When that happens the game disappears from your library and the seller is a username.
The practical question is not "is this store legitimate" but "what happens if this specific key stops working, and who is on the hook". Official resellers answer that with their own support. Marketplaces answer it with a dispute process against an individual seller, sometimes backed by an optional paid guarantee at checkout. Read which one you are buying into before you decide the price difference is worth it.
Matching the product before you pay
Most keys that "do not work" work perfectly — for a different product than the buyer thought they were getting. Listings are written by sellers, and the parts that matter are the parts most likely to be abbreviated away.
Check the launcher first. A game sold on several platforms has a separate key for each, and a Steam key does nothing in the Epic Games Store. Then check the edition: Standard, Deluxe, Gold, Ultimate and Complete are different SKUs at different prices, and a listing titled with the game name alone is usually the cheapest one. Then check whether you are buying the game at all — storefronts carry DLC, season passes, pre-order bonus packs and soundtrack add-ons under titles that read almost identically to the base game, and a bonus-content key will activate happily without giving you anything to play.
Finally, check the region. It is the field most often left blank in a listing and the one most likely to make an otherwise perfect key refuse to activate on your account. Our region locks guide covers what the labels mean.
Signals worth slowing down for
A price far below every other offer for the same edition is information, not an opportunity. When one listing is 70% under the pack and the rest cluster together, the outlier is usually a different region, a different edition, or a key whose provenance is the reason it is cheap.
Be wary of anything that is not a key. Listings offering an account login rather than a code — "shared account", "offline activation", "you get the credentials" — are selling access to somebody else's library. The password can be changed at any time, the publisher can lock the account, and you own nothing. Some of these listings are explicit about it in small print; some are not.
Treat "VPN required to activate" as a price you pay later. It means the key is bound to a region you are not in, and the workaround it asks you to perform is against the terms of every major launcher.
Look for a support channel that exists before you need it. A store with a visible refund policy, a contact route that is not only a web form, and terms stating what happens when a key fails, is a store that has thought about the case where something goes wrong. One offering none of those has decided that case is your problem.
What actually happens when a key fails
There are three distinct failures with different remedies, which is why "my key does not work" is not one problem.
The key is rejected at activation. Nothing has been consumed, and this is the best case. Usually it is a region mismatch or the wrong launcher. Screenshot the error, do not keep retrying — some launchers rate-limit failed activations — and open a ticket with the store quoting the exact message.
The key activates and the game is removed later. This is revocation, and it means the publisher decided the key should not have existed. Your recourse is entirely with the store, because the publisher will not restore a game to an account that did not pay it. This is where the difference between an official reseller and an individual marketplace seller stops being theoretical.
The key was already used. Codes are single-use, and a listing reselling a used code is either a mistake or a scam. Store dispute processes generally handle this well, because it is easy to prove.
In all three cases, act quickly. Refund and dispute windows are measured in days, not months, and a payment method with its own buyer protection is worth more than a promise on a product page. Keep the order confirmation and the activation error together — the two of them are the whole of your case.
The short version
Confirm the launcher, the edition, and whether the listing is the base game or an add-on. Confirm the region matches the country your account is registered in. Read what the store says it will do if the key fails, before you need it to do it. Prefer a payment method you can dispute. If a price is dramatically below every other offer, find out why before you decide it is luck.
None of this makes buying keys risky in general — most purchases are unremarkable. It makes the small number that go wrong survivable, which is the only part you control.