If it doesn’t run only offer $500. It could be as simple as $100 carb cleaning or as bad as a $1000 engine. Since it’s an unknown and the seller obviously isn’t getting it running to make more money you have to expect the worst so with the $500 and $1,000 engine you have $1500 into a ski you can sell for $1500. Any more than that and it doesn’t pencil out.
Also if it cranks I would check compression before buying. 150 psi is perfect 120 is time for a rebuild and more than a few psi difference between cylinders is a rebuild.