Bill of sale is optional in my opinion, I never really did it. I would always contact the DMV afterward so that they know the vehicle is no longer yours. Yes, sign the title over.. YOU write the mileage, selling price and all other information requested on the title. If you decide to give the guy/girl a break and let them fill in the price on the title for tax reasons, you should write up an honest bill of sale. Cashier's checks are just like cash so you shouldn't have a problem accepting them and cash itself.

Good luck w/ it all.