I can't really advise on how to get a good feel for a non-running ped but when I bought my Puch it had 5 thousand miles on it and had not been ridden for about 12 years. The piston was frozen in the cylinder, the carb was corroded shut and full of dirt, varnish and rust, both chains were petrified in their resting place, the tires were flat and the gas tank was full of rust and gummy residue. After a week of TLC and lots of penetrating oil the bike started after just a few trys. It cruises at 25 with no problem and I never replaced a part except for tires and a spark plug.
I think it helps that this was a ped from a good manufacture.
If it looks like it's all together there is a good chance it will be able to be revived but you have to be willing to fix it or have the money to pay some one to do so. If I had to pay someone to fix mine it would have cost many hundreds of dollars in labor alone! I was willing to gamble the $150.00 dollars purchase price because it looked like no one had butchered the bike. It's good to ask questions like, Do you know if the motor is siezed? Why are you selling it? Did it run up until the time you parked it? Do you know anything else that is wrong with it ?
With the ped on it's stand see if you can pedal the bike with and without the motor engaged (I have a lever on my Puch that will engage the motor) That may let you know if the piston is free in the cylinder. If you can't move the piston it is either corroded from sitting or seized from lack of lubrication.
Good luck and don't be afraid to offer them less than half of what they are asking, especially since it's not running.