Bring a compression tester, and see what you have. 120PSI or better.
Figure on a carb cleaning, new fuel filter/petcock/sparkplug. That should fire it. If not assume all the electrical is bad... Stator/cdi/rectifier/coil. That stuff is around 200-300.00. Deal to take that off the price.
It may just be a loose/bad ground or kill switch/kickstand switch/loose plug cap connection. don't try to fix it in front of the seller... Just offer on a worst case basis IF it has good compression. It is obviously worthless as a trade in (thus he still has it/new Vespa) not running so he will either take an educated offer or try to rip off the next guy.
If the compression is bad offer 150-200.00 or walk. Please keep us informed on what happens.
Welcome to the Forum as well!