Rule of thumb for old bike purchase....
If it's not a rare collector's item, then it's worth generally $1 a cc....IF you can ride it home!
If you can't, then deduct the cost to get it to that condition, and, at $70/hr shop labor, before long he will owe you to take it off his hands!
It pays to know ballpark repair cost ahead of time....
Carb job....$300+, tires $250+, Front brake job 100+, new tail lite $50, $70/ hr to sort out the electrics + parts $who knows......provided you can even find a shop that will take on the work!
But, the good news is a good frame with clear title is worth a hundred or two IF it is in HIS name!