Sales are governed by MSS 609.66 and 624.7132.
The long and short of it is that Randygmn's statement echoes my own. I have sold handguns and assault-style rifles privately, and every time I did it I looked at a DL and PTC (everyone had a PTC, but I'd look at a PTA as well) to cover my six. If I were to encounter someone who did not want to provide either of these documents, I would not make the sale. There's a lot to be said about due diligence and documenting with a decent bill of sale.