I think you made the right choice in keeping the Python and saving for the 686. At 80% finish you probably shouldn't worry too much about carrying the Python since that is what you want (especially since you plan to get the 686 eventually).
As for the $500 as a lowball price, I don't think the salesman didn't know what he was talking about, he was giving a price the shop might give for it (they will want to make a nice profit when they turn around to sell it). If they were giving the consignment asking price, they might be nuts. OK, it is more a shooter than a collector gun, but it is still a Python.