Winchester 1873 value?

Huh. It looks like it might have been nickel plated.

Need a whole lot more pics, details, condition of bore, Cal to even think about value.

yes better pictures are needed.

I think that “plating” wear is a good bet (chemical exposure would be my other guess).

This appears that it was nickel plated. Can't help with the value unless we know the details on the rest of the gun. Does the gun have the factory letter or is the serial number outside of that range? The screws are not buggered up so that is good. Notice that the wear on the receiver extends to the foream? Interesting.

I understand you've already passed on this particular rifle but for future reference, take a look at the screws on the underside aft of the carrier. These are for the carrier leaf springs. These will need to be removed or at least loosened significantly for a proper cleaning, (in your pictures they appear to have been turned at least a few times). If they ever look untouched on old rifles, take a closer look at the interior.