Besides the fact that keys on kinguin are in grey market area and can be revoked due to their possibly illegal nature...
Kinguin and 99% of other cheap sites are just key resellers, acting as middleman between person A and you.
person A might buy licenses with stolen credit card and resell them.
Credit company might revoke payment, keys get nullified, you end up with lost money and no valid license.
person A might "steal" licenses from their company (big companies have them by truckload) and once said theft is found out company/microsoft nullifies the licenses, you end up with lost money and no valid license.
person A might buy OEM licenses in bulk (computer shops can do this at decent discounts) these licenses CANNOT be resold (according to terms between MS and said company) except when preinstalled on sold computer. Once this is found out, yes.. license is revoked yet again and you stand to lose money and valid license.
In some cases, license might never be revoked.
In some cases, seller might sell a legit license.
Do you want to take the chance? I cannot suggest anything except "get genuine copy"
It doesn't really matter if key is OEM or not.
You can download and create bootable media from microsoft's own website
https://www.microsoft.com/en-gb/software-download/windows10