In the EU you can resell a software license under the UsedSoft principle (CJEU C-128/11): the original buyer must delete their copy and the buyer must take it over with the original key.
In the US the situation is murkier — most EULAs prohibit resale. But for boxed copies (Office 2021, Windows 11 OEM) FTC takes the buyer's side and resale is de facto legal.
Russia, Belarus, and Kazakhstan permit license transfer to a new owner provided the original copy is destroyed. The standard "first-sale doctrine" applies.
An HWID change is technically optional for resale but practically necessary: most software pins to hardware and demands reactivation when transferred. An HWID rollback to the new buyer's machine prevents activation issues.
Important: for an OEM Windows license, transfer along with the motherboard is allowed, but transferring "to a different PC" is not. Read your EULA carefully before reselling.