How do I pay?

Don’t do it.

Tell him to ship you the speakers first. Once you've received them, THEN you'll send him the cashiers check.

What he's asking for is basically the same thing - your complete, blind, trust.

+1 elrod

I'd pass on this deal.

If you must do it and can't do it in person, postal money orders offer some protection.  Crooks know that the feds will come after them if there is fraud involved and won't accept them. Ditto using postal mails to deliver it. Of course this guy might just be dumb and then you have nothing but a hassle on your hands getting the feds involved. My advise, don't do it.