I'm not sure but it looks like high KBB is around $9,000, you can send him an email with a picture of KBB private party sales and the list of options you used to get the price.
Offer him $9,600 that price is good for 5 days, if he fails to accept the price will be $9,300 another 5 days $9,000, another 5 days $8700, another 5 days $8400, down to $8000. I would make a schedule of prices you will offer on which date.
First only someone that really knows the value of his mods will even care what they are, but will they be willing to pay extra? The bottom line this truck is still a 2002 used truck. For $10k there are a lot of excellent later model used trucks you could buy. He is selling the truck for a reason, and most likely needs money. He will not get anyone buying at the price he is asking. If he really needs the money then he won't wait and take your high offer.
Just rem. if you don't have to buy right now because you need transportation, then there is always a better deal, you just have to keep searching and you can wait until you find the right deal.
Personally I want to save a couple of grand when buying used, and shop shop shop shop, then when I try to make a deal I try to figure out every possible angle to get the price down. I have the cash ready, so there is no delay. (Make sure someone is with you if you are going to take cash.) However I always have an expert mechanic go through the vehicle (at my expense, if there is any hesitancy I tell the owner he can have the report in case we don't make a deal, be sure to schedule this inspection in advance the day you look at the truck, and make sure the owner is ready ok with doing the inspection.
I did have one guy refuse to allow me to do the inspection, he was very comfortable with the condition of his vehicle. So I got him to agree that after I purchased the vehicle I had 48 hours to get a vehicle inspection and if there was anything discovered that would be considered a major repair he had to take back the vehicle and refund the cash. We put it in writing and made the deal. Note he also understood I was driving the vehicle straight to the mechanic shop. The reason he didn't want to allow the inspection, he felt his car was a hot commodity and others would want to see and drive the car, he didn't want to tie it up for a day and lose a possible buyer who might even offer more for the car. Since he had his own auto body shop he put in writing a 90 day warranty for anything up to $300 part cost/ his labor free. That deal was for $2700 2001 Hyundai Accent with only 19,000 miles the interior even smelled new.
Read my purchase of a 2002