I think your offer to take a form to your local repair shop to get
their opinion was very helpful and more than most would do.

The item was sold "as is" and he already left you feedback and paid
via money order. This is the best possible situation for a seller to
be in when they have done nothing wrong and a buyer starts making
demands.

If SquareTrade contacts you, respond that the item was sold "as is"
and you tested it before shipping. There is nothing they can do and
this is clearly his problem.

Good Luck!  I think he may just disappear when he discovers this
threats arn't working on you.