[Robust]

Depends, do you enter a username in and it activates it for them, or a HWID or something? If so, yes.

Otherwise,

Product Keys and Unlicensed Software
Product keys, key gens, serials and software cracks are not allowed. This includes the selling, trading, or requesting of any illegal unlicensed product keys. HF is not a warez forum and we will no longer tolerate our site being used as one.

from Disallowed BlackHat Activity

Without knowing how this is sold, my guess is no, it would also depend on their ToS on if this is allowed.

[Robust]

Arguably, it's a string of text that unlocks the signup form. Thanks for the reply.

Would I be allowed to sell an account, where someone tells me the username they want? Thus wouldn't handle the key, and the key wouldn't be sold. The account would.

Yes, I believe if you activated it onto an account and sold the account then it would be allowed in the NFA section.