I believe making it for resale without the proper mfg license is the crime.
Making yourself an unserialized AR from a 80% receiver is OK, later deciding to sell your unserialized AR is OK, but making ARs for the purpose of selling them is not OK.
Unless there's some sort of LE exemption that applies to these "buybacks", it might not be a good idea to make 20 receivers the day before a "buyback".
The "buyback" is usually "no questions asked / we promise you immunity", which protects you from state prosecution for stealing the gun or murdering somebody with it, but I don't think local/state police can grant you amnesty from feral laws.