Steam keys?

Legal though not always without risk

You are rightly aware that prices often vary very much, that can have many reasons but I now limit this to a very great simplification

Key reseller must of course buy their goods somewhere that can be done in the form of larger packages from the publishers are sometimes global versions even cheaper also the tax plays something with you may have noticed that some pages in the cash register suddenly demand a VAT

Unfortunately, there are also platforms that cover their product codes from credit card fraud such product code can be deactivated afterwards in this case your game on Steam would suddenly disappear from your library