How do I pay?

Personally, I wouldn't do it.

You would only want to purchase those if you can inspect the speakers in person and pick them up yourself when delivering the check. Think of it as exchanging prisoners at Checkpoint Charlie.

If you just mail off a check you are at the complete mercy of the seller's honesty, or lack thereof.

Isn’t there some sort of escrow service?

That's paypal. If he doesn't want to use them, he probably doesn't want to go that route at all. Tempting as it might be, I wouldn't do it with no feedback.