The Marlin Firearms Company has in one way or another been in business for over 140 years. Throughout that time, one thing has remained constant: the production of lever action rifles. With literally 30 million of these guns out there floating around, there are a few things to look for in used ones.
Stock and forearm
This 'bullseye' insert will let you know whether a rifle stock is made of birch or walnut and also give you a clue as to when it was made.
Unless you are getting a super-cheap great deal, or are looking for a woodworking project for the weekends, stock and forearm issues are a pain. Many fifty-year-old Marlins have been carried in the woods for a fifty seasons and have the battle scars to show it. Banging on tailgates, tree trunks, and fence posts will take its toll on the walnut in a heartbeat. Dents, scratches, and dimples are to be expected and can often be smoothed out later with a steam or oven treatments so don't let those worry you. What you do want to look for are cracks and splits. The most common places for these are at the buttstock at the thin handguard, and the buttplate. A hairline crack, large chip, or a split-- if not properly repaired-- can lead to an expensive replacement.
Used Marlins pop up everyday on Armslist, Gunbroker, GunAmerica and other sites online as well as haunt almost every pawnshop, gunstore and classified ad board in the country.
If you do nothing else, check the action of the rifle itself a few times. Work it open and closed and pay attention that all of the parts move smoothly without binding or catching. If it wiggles, seems bent, or is very loose in ways not intended originally, you may have an issue. Worse, it may be a rifle that has been repaired by an incompetent or ill-trained gunsmith. Most Marlin lever actions of almost any year will take 5000+ rounds before needing a repair to the action. With a typical hunter/owner firing a box or two a year through it, this translates into an expected lifespan of about 125 years or so. With that in mind, odds are the action should have a lot of life left in it.
Get a light of some sort. Any small strong bright flashlight will work but most sporting goods stores/gun shops have for use or will sell you for about $7 a bore light. This sharpie sized penlight has a hook-shaped tube that you can put inside the chamber throat (make sure it's unloaded!) to shine up the barrel. Look down the muzzle and into the barrel. If you see nice smooth and crisp rifling then you have a winner. If you see rusty patches, blotchy looking metal, or cracks that look like wrinkles, the gun may not be shootable. Ask if you/they could clean the barrel to see if it's just debris but if it's no better after a cleaning, you may want to rethink your offer.
Collectability over usability
When looking at a used Marlin, examine the model number, caliber, and serial number style. Very old Marlin lever guns (made before 1948) are often extremely collectible if they are in good, unaltered condition. Guns made before 2010 will be marked Marlin Firearms, North Haven Connecticut and have a 'JM' proof mark on the left side of the barrel by the receiver. Newer guns made by Remington in New York since then will have a 'REP' mark instead of a 'JM' .If you see the white circular insert of commonly known as a 'Bullseye', the gun was made after 1922-29 or after 1946, and denotes a walnut rather than birch stock.
Keep in mind that some models, such as the 1894 have been in near constant production for more than a century with dozens of subvariants made. Some of these variants are worth thousands. While these obviously wouldn't be shooters, they can be good investments.
There is a quick cheat to tell the age of a Marlin lever gun by the serial number. Guns manufactured from 1948 to 1968 will have one or two-letter code was used to designate the year of manufacture. All Marlins manufactured from 1969 to 1990: The first two digits of the serial number designate the year of manufacture, either as the last two digits of the year (in 1969-71) or as a number code (1971 and later).
Guns outside of this ranges mentioned above will be likely made 1881-1947 and are antique rifles with possibly high collector interest as relics. These will often be in ancient calibers such as 32 Special and .38-55. They should not be shot until inspected by a competent gunsmith. Their value can be widely subjective and vary widely from a few hundred to several thousands of dollars and a firearms appraiser should be enlisted to give a detailed letter. A good place to start your homework is the book Marlin Firearms by William S. Brophy.
Regardless though, it's unlikely that you will come out on the bad end of the stick if you follow a few simple tricks. After all, 30 million Marlin owners can't be wrong.