What is the value of a 38-55 Marlin 1888 with no original finish but otherwise good condition?

The 1888 was never made in .38-55. If your gun is marked 38W, then it is a .38-40, if it's marked .38-55, then it is not an 1888, but probably a 1893. No way to give you a value, as I don't know what you've got, nor could I tell you without a few pictures of the gun. The 1888 was only offered in .44-40, .38-40, and .32-20, not .38-55. Values depend on the model, and condition. If your gun is a .38-55, it is not an 1888, so I don't know what model you have.