Used bags are not worth a lot IMO since the reproductions are out now.
However, someone with a survivor may want it since it will have the same patina and age to match versus a new one which would stick out, but that buyer may be even harder to find than someone who is restoring or would just as soon have a new one.
As was mentioned, there are a few variations of these on different Ford vehicles which also include a screw on and flip fill cap, different sizes, logo printing and also red in addition to the blue and black mentioned.
The originals I have seen on Broncos have been blue with a screw on cap and said FoMoCo on them.