He probably means low compression on the rear rotor, in which case he will need a rebuild. He should still be able to get it started though. If the car has been sitting for an extended amount of time, it has likely lost compression on both rotors. This can usually be remedied by squirting automatic tranny fluid or marvel mystery oil through the leading plug holes while turning the motor over by hand. Put a few ounces in and mix it in real well. Let it sit for AT LEAST 24 hours before attempting to start the car. You may have to repeat this more than once and let it sit longer the second time. If it doesn't work, the motor will probably need to be rebuilt. If it does work, the seller will thank you for your help, send you on your way, and up the price of the car, or keep it, so either way your screwed. If the motor is in otherwise good shape, the rebuild will be a LOT cheaper than if the internals are worn or trashed. How much is he asking for it? If the price is right, it may be worth your while.