I'll be honest- I know nothing about cars, but I'll be damned if I ever pay that much on an 11 year old FORD pickup with 100k+ miles.
I'd say no to that (unless the tires and sub are something you're seriously interested in).
If you're interested, go for it, but try and get the price down.
My time as a public marketing person would say this:
Visit the guy, get excited about the truck.
You know you want this truck (even if you don't). Let that excitement be visible.
Open body language, always smile.
In a firm, commanding voice, ask what the price is, but still make sure you're smiling and making direct eye contact while doing this.
When he throws that 15k figure at you, break eye contact and close body language.
If you've been doing this thing right, he'll already have thought he sold you on it, and this should throw him. You want him to get worried.
"Well. I looked at KBB and this truck was valued around 12 grand. You've got some aftermarket stuff on it and new belts, so lets meet at 13 grand."
Assert that you are doing him a favor with this offer and that you are setting the price. He knows 15k is steep and he's hoping you'll roll over and deal with it.
When you're giving him that figure (or whatever figure you think is good), make sure you're making 100% steady eye contact and smiling.
You'll also need to say this as if you know he'll take your figure. Confidence in yourself is the most important thing here.
So long as you do all of that, you'll gain an extra 20-25% chance he'll knock the price down.
There's always that chance he's strong-willed and won't do it, but from my experience, anything is possible if you push just right.
Boom, enjoy your new truck.