MikeS
That's a tough one. If you can find someone looking for a .357 Supermag you may do ok. AFAIK no company is making .357 Max ammo anymore, chances are it's going to be a hard sell. The .357 Supermag without .357 Max ammo is just a jumbo .357 Mag. I recently bought 500 rounds of .357 Max brass just to cover myself.
As a benchmark, about a year ago I found a lnib original Monson, Mass. 8" .357 Supermag with the original 2 barrels, (same length, DW originally sold the gun with 2 barrels as a response to the forcing cone erosion problem created by the .357 Max.) 2 sets of grips, all paperwork, and a Burris 2X scope mounted on it. Total price $300 out the door. Did I get a steal, maybe, but long range silhouette guns really are not in huge demand.
To answer your question, I honestly do not know what the Supermag would sell for in your market or on the Internet. My advice would be to ask about $550 and see if there is any interest. Adjust price accordingly.
Good Luck
Take Care