Check for rust, especially in the barrel, and look for muzzle wear. Generally, rifles should be cleaned from breech to muzzle, but it's difficult to do this with a lever gun because it's difficult to remove the bolt, so many people clean lever guns from the muzzle to the breech. If the person doing the cleaning uses a crappy cleaning rod and/or fails to use a bore guide, this can damage the muzzle, which can make the gun less accurate. A good gunsmith can fix this by redoing the crown, but it's not exactly cheap.
Beyond that, I'm not really familiar with Marlin rifles, so hopefully someone who is can give a few more pointers.