Anywhere from 200 to 500 depending on condition
I just bought one from a private party for $75.
In good working condition $150 to $200
If your model 55 shotgun is the regular shotgun with a 26in-28in barrel,then it is worth between 30-100 dollars.If it is the marlin goose gun with a 36in barrel they are going for between 90-200 dollars depending on the condition it is in.
The price range on your shotgun is between 165-200 dollars depending on the amount of finish being between 60%-90%.The Marlin model 120 magnum shotgun was made from 1971-1985.
Between 140-200 dollars,depending on the overall condition of your shotgun,and a decent bore.
I presume you are asking about a Marlin model 120 pump action shotgun. In good condition a standard 120 is worth about $200-$250. If you are asking about The Revelation model 120 that was manufactured for Western Auto Supply by Marlin, then you are looking at a generic model 60. Worth about $75-$125 dependent on condition.
The Marlin model 336 will sell for between 200-350 dollars,depending on overall condition.