Money orders are risky. Not cashier'* checks.
Go to your bank and get a certified check. A certified check is GUARANTEED FUNDS at the time of check cut. A hold is put on your account for the amount of the check so you can't withdraw a large chunk of the money. The check is guaranteed to clear because of that hold. I'm surprised the seller doesn't know this.
I'd only pay with a certified check or cash. It protects him, more than you.
For his protection, don't get money orders. Banks frown on cashing them because there'* so much fraud involved, and because banks seldom are ever able to recover the losses.