They are stores that have to pay taxes, respect and comply with the laws, and reach agreements with the developers to sell their games. Basically the same thing that any company does that wants to sell third party products.

Stores or marketplaces such as G2A, CDKeys or Eneba are established in countries with a lax legality to operate without worrying about laws that protect users or developers, although they may have cheap prices after they compensate it with extra fees, shields that do not protect, charging for inactive accounts or even stealing game keys from developers.

A lot, from this website I put the best deals that I consider most relevant and work for Europe. But there are more legal stores that operate exclusively for regions such as China, Russia, South America or North America.

Easy, the same patterns are always repeated in stores that do not operate legally:

• The store is not legal if it sells Blizzard games, this company only sells its games in digital format in its own store and Amazon.
• The store is not legal if you sell Steam games as a gift.
• The store is not legal if they sell wallet cards for Steam or other platforms.

For two reasons, the first is money. Illegal stores have invested in a lot of advertising over the years to buy spaces in any media related to video games (press, streamers, YouTube, …). Very few people and developers speak freely about these stores for retaliation from the sold media.

And the second reason is the tremendous lack of professionalism seen in many media when talking about the PC, for example, when Epic launched its platform it was easy to find news referring to a lack of competition in the prices of games for Steam.

Everyone wants cheap games, but I think everyone also wants to earn money for their work. I personally recommend using piracy before buying a game in an illegal store, the developer will see the same money (nothing) and you will save money.