Do: Accept cash and meet the buyer at your bank
[NOTE: This isn't exactly about maximizing your car's selling price, but if you get scammed, your selling price equates to zero, so it's still crucial advice.]
When using cash, your bank will protect you in two unique ways. First, the bank can verify the money isn’t counterfeit. Second, the bank's security team will have a video of the transaction. I have shaken hands with several people who were all too willing to buy my car, until they realized a visit to a bank would be required. Then they "got hungry" real quick and they thankfully left my life forever.
Don’t: Accept a personal check. Ever!
A buyer might be understandably leery of walking around with tens of thousands of dollars in cash, and that's OK. If it’s a higher dollar amount, you can accept a certified check, but you absolutely must verify it by visiting that bank in person and cashing it immediately. If the certified check is from a non-local bank, call the bank to verify it’s good, and then don’t relinquish the title until the check clears and your bank confirms that the funds are physically there. There should be no exceptions to this rule.