Maybe my track on the 19 snubbie will help:
Smith & Wesson 19-3 2-1/2 inch barrel. Revolver in .357 Magnum,.38 Special, Plus P. Bluebook of Gun Values (33nd Edition) indicates between $325 in 95% condition and $550 in 100% condition for this gun. I have 64 recorded prices for this model gun from $395 to $995 over a period of 2 Yrs 11 Mos ending March 18, 2013. The average price as of that date was $567. No refurbs, no refinishes, regular sights. All represented as 95% or better. No distinction for import location or engineering revision, if any, no special editions, no distinctions for box, papers, tools. Current or last MSRP is $457.
You can see from the data chart that the Average Price Trend is at about $630 as of this month. Better examples of the model would command higher prices. So his 686 and some cash would seem to make it even at the price he's offered you, if you wanted to press the point.
The 686 he has is not "New In Box" unless he has an FFL and the gun has never before been sold at retail. That's a legal definition and influences warranty eligibility. It is ANIB (As New In Box) if it has case, papers, tools, warranty card, etc. Otherwise it's a used firearm
The 19 is more concealable than the 686. The 686 is large enough to be difficult to conceal. Some 686s have seven round capacity, those bring a bit more than the six rounders.
I have both but I'd sell my 686P 2-1/2 incher I purchased new before I'd sell my 19 I purchased used: