of course it's possible,each cylinder has a round ring that seals the two mated surfaces(head to block),if you break the integrity of the ring it allows leakage of compression on that cylinder....the other sealed cylinders are unnaffected.(this i know from experience by the way)
it is possible that the piston ring may be cracked,you can exclude this by squirting some oil in the spark plug hole and immediately doing a compression test,if the compression goes up the chances are it's a piston/ring problem
it is possible that the compression is leaking into the water jacket via the headgasket,would get worse with more boost,may show up by overheating,pressurising the cooling system,rough idle on start up first thing in the morning(water leaks into the cylinder overnight and fouls the plug)
if it's a small leak it is unlikely to account for a big drop in compression,best way is to test it with an S-TEK tester that is very sensitive to carbon monoxide in the coolant(even if there are no symptoms),costs about $80 from auto parts store eg coventrys
if its a burnt valve you may hear it popping from the exhaust on heavy decelleration
obviously head off is the best diagnostic route,but the above would take less than 1/2 hour to perform and give some indication of the problem
good luck with the sale