Yes, bill of sale. Google a generic one and fill it out, take two copies one for him and one for you. That should suffice for temporary tags (or just keep it in the vehicle if Wisconsin is as easy going as Iowa).
Bought many cars that way, never had a problem, but best bet is to call your county registration group and ask them to be safe.
Rover - 2013 Holden Commodore SV6 Sedan
Clark - 2013 Holden Commodore Sport SV6 Wagon
Brumby - 1955 Holden FJ Ute