Hi Korivian
The licences those websites and resellers are selling are volume licences, and while they will initially activate, that activation will run out, because a volume licence needs to be re-armed form a company Key Management Server on a regular basis . . .
Honestly, as you say, 'If it sounds to good to be true . .. ' those licences are a waste of money
There is no 'cheap' Windows 10 licences, sorry, the best and only real place to purchase Windows is from Microsoft, that is the only way to know that you are getting the real product . . .
A further thought, you are building a PC for your Wife, does the company you are purchasing the parts from sell an OEM licence, which you would legally be allowed to use on a new PC, since you are not keeping that PC yourself . . .