This is a complicated thing to answer. I imagine several posts are being composed as I write this.
Does the engine turn over easily?
Two screws to take off flywheel cover. Take it off and turn engine over by hand. It should be smooth. You should have difficulty getting past the compression as the piston travels to the head.
While you’re turning the flywheel, go back and forth a few times. You don’t want to hear clicking or slack in the crank bearing, connecting rod stuff. I guess thats not a perfect description, but if it’s sounds really rough, I’d argue that with the seller.
If the internals of the engine are not ruined and the thing just won’t start, it shouldn’t be very expensive to get it going. Maybe free if it just needs cleaned and tuned.
Price range varies wildly. Is the asking price over $500? Can you post a pic? Moped price is kinda how bad you want it. I never paid more than $100 for a moped.