Does a dealer have to do a NICS check when selling a used gun

[Mr. 16 gauge]

I think that the key word here is "Dealer"....I have bought several used firearms (both long arms and handguns) from dealers and each time I had to wait to pass the back ground check. My most recent purchase (a couple of weeks ago) was a long arm, so that is about as up to date as I know of.
Private individuals are a different matter....here in MI, long arms are traded/sold without any kind of check. Handguns require either a pistol purchase permit (obtained from the local PD or Sheriff) or a valid CCW, even for private transactions.
I can't speak for the private sales in other states.....