1. Ride along on the test ride is appropriate. If they want to discuss something in private, they can stop along the way to do so.
2. If they write a check, have them do so at YOUR bank. Your bank will contact their bank to verify funds and have the funds wired immediately.
3. If they are financing the purchase, plan to work with their loan officer to conduct the transaction in a manner safe for both parties. If they are approved for a loan from a reputable place, they will have already signed a power of attorney for the loan officer to conduct the transaction on the purchasers behalf.