Here's the way I look at it, if it's rust free, runs perfect, ALL options work, and it needs no repairs, then it's fair price. If any of those conditions aren't met then the price has to come down accordingly... Could you duplicate this truck as it sits for that amount of money? Probably not.
Could he have put $4300 into it? Very easily! He's had it over ten years, it has been modified as well as repaired over those years. I've had mine for 6 years and have almost $9000 in mine if I add up every nickel and dime I've put into it... Let's not forget, the balls in his court. He has a truck that he isn't looking to sell, that you want to buy. Just looking at the pics it appears to have had alot of work. Look how clean that frame looks in the one picture. Granted, pictures always look better than in person, but it looks like he has put a lot of money and hard work into that old blazer.
I started with a low mileage, $1300 rusty blazer 6 years ago, and I'm way beyond what he quoted you, so to me it's not that bad of a deal... Would he have some trouble selling it for that price? Probably, but it might go for close to that cleaned up and listed on eBay. The question is, is it as nice as the pictures? The other side of this is, he would probably need to replace this vehicle, it looks like he tows with it. So why would he just give it away? He probably has a little wiggle room at that price, but be careful not to tick him off if you really want to buy it.