There is no way to guess what is wrong until you actually tear the engine down. It could be a 5.00 valve seal and then again, it could be a 300.00 complete rebuild and bore job. [or more]
300EX's in GOOD shape go for that price all the time. I would skip this one and keep checking around or talk this guy down a few hundred dollars so that you have a little room for engine work without going over the value of the machine.