If he says it needs the valves adjusted, then the reality is it needs the valves replaced. Plan on spending around $500 to properly service the head. Unless it's been done recently it will probably need piston and rings at the same time. You'll also need to find out if he's changed the cam chain tensioner and the water pump shaft, seals, and bearings.
KBB puts it's value at $1900-2700 assuming perfect mechanical condition.
It's a great bike (I've kept my 2002 CRF since day one) but it needs to be maintained properly. Lots of riders sell the bike when it's clear it needs work. Buyer beware. ;)