Decoys as noted depend upon who & how they were done. A known carver will get big bucks,,, as will antique ones. Mass produced, or more common ones won't bring much,,, but as mentioned,,, sleezbay & other auction sites can get you the most money. The devil is in the details. Markings, dates, signatures are what get buyer's attention.
As for the mounts. In NC,, and in many states,,, it's illegal to sell any wild game parts,,,including mounts,,, UNLESS certified by a taxidermist. I'd discuss it with the state game commission before attempting to sell any same mount,,, even fish.