i would tell him so sad to bad, all privete party sales unless otherwise stated are as is. if you knew the headgasket was blown when you sold it then i would feel morally obligated to try to take care of it, but thats why its always recomened that if you arnt entirly competent with automobiles that you have a vehical checked out by a shop before buying. at the most i would maby offer him $250 back to go towards the replacement of the head gasket. but that is also dependent on how much they paid for it, if they only paid 500 i would tell them well what do you expect for a 500 dollar vehical, if they paid 2-3k for it then 250 would be a fair compromise to me. a head gasket replacement in a shop would probably run in the 500-600 dollar range (i pulled that number out of my ass because i do all my work myself but a head gasket is $25 and probably 8 hours of labor assuming the head is not warped, if it is a machine shop will probably charge about 25-30 to resurface. iv got an head in the mechine shop its costing me 250 to get the head magnafluxed re surfaced valve guides valve guide seals new valves (they were warn beyond the point of mechining) and mechining of the seats. well anywhay sorry for the rambling i just got off a 13 hour shift