No registration or anything like that in Michigan for long guns. As long as you do not devote time and interest to selling firearms as predominant form of "income", you do not need a license. You would be smart to ask for a copy of their driver's license so that you know that they are 18+ and a resident of Michigan. I would also consider having something typed up that the person signs attesting to the fact that they are not prohibited from purchasing a firearm under Federal Law or Michigan Law.
You can put an ad in the local paper or go to michiganopencarry and look for the firearms for sale section. Many other websites also have sales areas, MCGRO, Michigan Gun Owners, gunsamerica.com; gunbroker.com. Just limit sales to those in Michigan and then you don't have to go through a Federal Firearm Licensee. If you sell to an out of state person on gunsamerica.com or gunbroker.com, there are directions there as to how to do that as then the buyer must go through an FFL.
If I have forgotten or misstated anything, I am sure someone here will chime in...