Is it legal to buy OEM kexs from third party sites?

[Alberrtros]

Please help me with this. Thank you

 

[Grizzly]

If you do not get the CoA with it, I would not buy them. Those keys may be only temporary keys that work for a while then get disabled.
Been there, done that, especially if they send the key via email only

 

[Bighorn]

It used to be that a branded CD for installing Windows couldn't be used on a different brand computer, haven't had occasion to install on a custom computer but would guess the same applies with OEM keys. Generally, beginning with Win8, the required license information is embedded on OEM motherboards.
caveat emptor