What is your height, weight?
What do you want the bike for? Commuting, joyride, touring...
Basics...(Things I wish I had done)
1. Check engine compression.
2. Check to make sure all lights and electrics work properly
3. Go through wire harness as much as possible and look for damage
4. Check the tires for age and wear
5. Check the brakes for proper function and wear
6. Look in the air box/filter assembly...things tend to be missing from here
7. Perform a Spark test...or atleast check the exhaust pipes to make sure all are getting hot
8. Check inside of gas tank for rust
9. Test ride the bike...take it all the way to fifth gear if you can safely...if the clutch is slipping it will show it in the higher gears first.
10. ask history questions...what if any maintenance has been done on the bike...who did the maintenance and is there any record of it.
11. Make sure the bike has a clean title.
12. Get a mechanic to look at the bike if you don't feel competent.
This may seem excessive but trust me when I say that it's better to find out that your wiring harness is fried, your carbs need to be cleaned, your front brake locks, you need a new rear tire, your fuse box is melted, etc... before you pay 1500 for the bike. All of these things are resolvable problems, but each problem decreases the value.
Oh and don't take the PO's word for it...no matter what it may be.