Idk why difference but $3k+ for 20 ye old car is crazy, I'd offer him 1 grand or less and gauge the reaction. I bought and '07 with 140k for 3 grand this spring, its still running excellent with just standard maintenance. Good luck
Edit: looked at site, u sure kbb price isn't for private party and not retail (dealer) like nada
NADA is a dealership retail price, which allows for profit after acquisition, cleanup/refurbishment, and overhead/sales expenses. The KBB prices you provided are for private party sales; they only list one retail price.
To cmpare apples to apples, look at NADA and KBB retail.