Both of you at his bank. Have the buyer get a cashier's check from the bank teller, and you witness the transaction. Buyer gives you the cashier's check. (The buyer's personal check doesn't enter the equation as far as you are concerned.)
The key point is that cashiers checks can be fake or stolen, and you want to witness that it came from the bank. If the buyer should show up with the cashiers check, I would verify it at the issuing bank before signing over the title.
>$15,000 in cash is inconvenient to carry around.