Lorder, I don't know anything about the Marshall gun, but Ithaca double guns have a following, and some of them can be worth some money. Ithaca produced several doubles over the years with model names like Flues, Crass and NID. How much your gun is worth depends on several factors such as what model it is, what grade it is (if it says Sousa grade you are in luck) and, most importantly, its current condition. Collectors of American shotguns are fanatical about factory original condition, so a gun that has been altered or refinished loses significant value. A well used but original condition example is still a desirable gun.
If you can post a picture of the gun the model and the grade can be determined. Then and only then can a value be determined. Almost any Ithaca double will find a buyer, even if only as a parts gun.
Sharptail