Overall condition?
(factor cost to bring it to stock)

How hard is it to start? (Factor cost to get it to start first or second kick.)
(can they even get it started and prove it runs. Dont buy the "just needs the carb cleaned")

How well does it run?
Check compression and crank bearing play with flywheel wiggle.(Factor new top/bottom end, if needed.)

Is there a valid title? (cost to transfer title)

Currently registered or back fees? (factor cost to bring current)

What condition is the wiring? (factor rewiring)

Condition of the plastics, tires, axle, bearings?

There is usually a lot of room to negotiate when sticking to the big items while pointing out all the work that will need to be done. Leverage can be made by acting more disinterested as the deal goes on... Especially if it's been up for sale for awhile.

By reminding the owner of all the work that will need to be done, it makes it easier to get a fair price.