As long as you're selling to a WI resident, there's no need to go through a licensed dealer. No paperwork, no background checks.
I'd suggest that you ask to see a driver's license, though, and write down the buyer's name and address. Years back, a friend of mine had a sheriff knock on his door. A gun he had sold years before had been used in a crime, and was traced to my friend. Fortunately, he had the buyer's name and address and a bill of sale.