How do I figure out what to ask for a used gas forge?

One last tip, mention the lining condition and burner performance in the listing itself, not just the photos. People who've never owned one worry about hidden damage, and a line like 'burner lights first try, holds steady, lining solid' answers the question that kills most deals.

On the tank, I'd keep it unless a buyer asks. Partially full propane tanks are the kind of thing that makes pickup day annoying, and most people into casting alredy have one kicking around from a grill or a torch setup. When I sold my old kiln I went local pickup only, and honestly that was the right call, because somethig that heavy gets beaten up badly in shipping and the ceramic inside can crack if it gets knocked around in transit. As for the rust, a quick once-over with a wire brush and it'll photograph like it never happened.

Before you take photos, hit that rust with a wire cup brush on a drill and wipe it down with some oil. I did that on a bench grinder I sold last spring and it went from looking neglected to looking like a tool someone actually cared about. Took maybe twenty minutes and I got full asking price without haggling. On pricing, check what these furnaces go for new, then knock off about a third if the refractory lining is still in good shape. Sitting unused for a couple years doesn't hurt these much, honestly. If anything, a furnace with low hours is a selling point, not a flaw.