So what do you do if you bought one when the long gun registry was in effect and sold it in the EE since? Seriously. I had one and didn't keep a record and will probably get the letter.
If you get a letter, you first need to question how you could get one. If you do get one, and I suspect you won't because they know we will all be waiting for that first one to come so the court case can start, call them and say that you sold it after the registry was gone, to a licensed owner and that you don't know who it is as you checked the license upon selling and are not required to keep a record. If that is not enough for them and they want to come and check, refuse. Then make them get a warrant and have your lawyer there ready to serve them with charges of abuse of power.