You can legally ship a long gun or bare receiver through USPS. It MUST go to an FFL in the buyers state of residence. Make sure the FFL will accept transfers from individuals, some do and some do not. It is perfectly legal to ship as an individual to an FFL, I have done it dozens of times.

If it is going to a resident of the same state you are a resident of you do not need to ship to an FFL. In state sales, even those going through the mail, are treated like a face to face transaction.