I've got a LOT of thoughts. First, a 19-3 is a terrific gun. Long time/hardcore S&W guys will tell you that these Bangor-Punta era guns were not as well made as the earlier guns, but most will agree that these are -so- much better than current guns. And keep in mind that Smith & Wesson no longer makes any K-frame .357 Magnum revolvers.
The K-frame has a fantastic "size - feel" to them. If your plan for the gun is a lifetime of service with .38 Special ammo and also the ability to shoot .357 when you choose, a K-frame will be perfect. However, if you want to thrash the revolver with heavy magnum loads on a regular basis, there are MUCH better choices than a K-frame... as it will only take so much of a beating where a GP-100 or an L-frame (or N-frame or Redhawk!) would take a lot more.
I love the Model 19. I would take a 19-3 in a heartbeat. (I have one that I adore) The price of $700 would be on the far upper end in my area (in my opinion) but it's not CRAZY if the revolver is mint in "almost unused condition."
Here is what I'm am 100% sure of: I'd much rather have a very good 19-3 than a box fresh, never been touched and brand spanking new 686. These current guns have dropped off, IMO, and yes, I know that not everyone agrees. As I own more than a dozen different revolvers, I can tell you that my opinion has been formed over 25 years and dozens of revolvers owned, handled, cared for, and shot a lot. I own a 686-3 ('88) and I'm familiar enough with new 686's to have formed this opinion and am firm in my stance.
I would try to get him to come a bit lower than his $700, but if he wouldn't waver from it -AND- it's as "nearly new" as you've been led to believe, I'd rather own it than any of the four listed above, assuming the 686 in question is new production or within the last 10 years or so.
Attention Brass rats and other reloaders: I really need .327 Federal Magnum brass, no lot size too small. Tell me what caliber you need and I'll see what I have to swap. PM me and we'll discuss.