So you bought a 1500.00 car... In a state where 1500.00 won't buy much of a car at all (here in mi used car prices are significantly lower, and 1500 buys a barely running needs work kind of car)
Now it needs a HG... First Head Gasket failure can just happen, it doesn't mean that it was leaking the day before. You can try a bottle of BARS stop leak available at any auto parts store. Buy the head gasket/block sealer type. Don't buy blue devil, it's junk.
What kind of car is this. Typically Head Gasket repair is going to run 1400-1800 but some makes may actually be more. I don't know that I would invest that kind of money into a 1500.00 car, so I would try the stop leak route first.
Used car sales in VT are as is when its private party to private party. If he holds a dealers license you may have recourse. Lots of mechanics do get dealers licenses, if he has one you can make some claims, but would have to prove that he KNEW that the Head Gasket was bad prior to the sale of the car.
How have you come to the understanding that the Head Gasket is to blame for what I can only assume is an overheat issue? Maybe it is a hose, or a radiator, or a clogged heater core, has a repair facility made this determination?