How do I pay?

Here are 3 online escrow companies licensed in California. Does not mean they're good just that they are licensed. 

https://dfpi.ca.gov/online-escrow-companies/

I wouldn't do the transaction otherwise. 

There a a lot of speakers you can buy for $4K. If the person is being difficult buy something else. Especially for someone with no Feedback. This seller should jump through hoops to get a sale and some good Feedback.

It’s sad two parties can’t do a gentlemans agreement these days.

The honor system is a risky thing. I've done it in the past, but wouldn't now. 

Just send the seller a quick email asking him if he'd accept UPS COD or an escrow service. I suspect his answer will be "no"...

Gee, I'm sorry but I found a better deal.