If you are going to sell it or possess it in public you will need a serial number if there is not one.
I would not know anyone who would touch it if there is no serial number on it.
If you are the one holding the bag it is you who will catch the felony case.
Welcome to Marlin Owners.
The serial number is on the inside top of the receiver. It also can be found on the side of the lower tang, you might have to remove the stock to see that one. As to price, it all depends on condition. Outside and what shape the bore is in.
Many firearms did not have mandated serial numbers until after 1963 or so IIRC. You can research that date. But I have several pre 1960's USA made firearms with no serial numbers. A photo of your firearm would help us identify and date it for you. Welcome to Marlin Owners.