Check out a copy of Fjestad's Blue Book of Gun Values in your local library or book store. It will give you current values for old Marlin pump action shotguns.
The Marlin firearms company produced pump action shotguns from 1898(the Marlin model 1898)all the way up to 1985(The model 120 Magnum shotgun).during that time span Marlin made 17 different model pump action shotguns.
You would need to include the model of marlin shotgun you have and a detailed description of the shotguns overall condition to get a correct answer to your question.
The marlin 1897 lever action 22 is valued at $555 in fair condition. In excellent condition, it is valued at $1,150.
The Marlin 22 1929 39A lever action in good condition is valued at $400. In excellent condition, it is valued at $600.
The Marlin lever action 38w, serial number 41436 is valued at $700 in excellent condition. It is valued at $500 in fair condition.
Slide action shotguns with the name CHICAGO ARMS CO were made by the Marlin Fire Arms Company for and were sold by either the Fred Biffar & Co or Sears Roebuck & Co both of Chicago,IL. The gun is the same as Marlin Model 26A. Do not attempt to shoot this gun!!! Marlin has put out a company bulletin advising that these old Marlin slide action shotguns are not safe to shoot.
Depending on condition, a Marlin 1895 lever action in 45-70 sells for $400-$500.
The value of a Marlin Cal bolt action model 187 gun depends on its condition. This gun in excellent condition is worth between 90.00 and 135.00 as of 2014.
marlin 12 gauge bolt action goose guns sell for about $200 in great condition
Your Marlin model 336 in the condition that you describe will bring between 300-375 dollars.
Marlin Model 59 were single shot blt action shotguns that were produced in .410 only. Marlin manufactured less than 50,000 units. Value at 95% is about $150 to $175. I believe they were all choked in Full.
your marlin is valued at between 200-350 dollars depending on the overall condition of your rifle.