Condition is everything Rondog, value depends upon condition. Search the web to learn how to evaluate a used revolver, and first things first, make sure the revolver is not worn out or nearing the end of its life.
Second, though you may not have the time, watch auctions on gunbroker, see what 686 and 586 L frame revolvers with 6 inch barrels command from bidders. Value of something is whatever the market will bear, value is whatever a bidder will pay.
It's very good that you can evaluate the gun in person. Do your research on how to evaluate a used revolver, make a checklist, and go see the gun. It can't hurt to negotiate a lower price but do it after you see the gun. I've seen guns that were so nice, and the price so fair, I would have been a jackass to make a lower offer so I just paid what the gun was worth. Nothing wrong with giving a man what his item is worth.
Best of luck determining what it is worth, I myself know little about L frame values but my advice above is sound. Let us know how it works out and I'm sure we'd all enjoy seeing the photos if and when you buy it!