In Wis dirtbikes are not titled but my guess is if you have a title for the bike be sure you sign it over to the buyer once payment is confirmed.
I would also have a receipt with the wording somewhat to the effect whether there is a title or not just for my own records.
I __Your name_____________ have sold this __Year make and model ____ Veh ID Number___ ___________ to ________Buyer_name and address________. This bike is used and sold in as is condition with no warranty implied or given.
Payment is ___$amount___ by ___cash/check money order#___
signature seller ___________ date__________
Signature buyer____________ date__________
Not a total legal document but it gives you the date sold, name and address of buyer, and his signature that he knows it is used with no warranty.
Make it duplicate so he gets a copy and you keep one also