I've recently ran across a chance to buy a model 659 and was wondering if someone could tell me a fair price I might expect to pay.  All I know so far is that it is in good shape and was originally bought around 1984.  Any help would be great.

I would check a couple sites like gunsamerica to get a baseline. Check it over good to make sure it's functioning right. High chance it's a PD trade in, maybe not since when that was made, the semi choices were slim with the wonder nines.
couple things, cock it, make sure the hammer stays cocked(decocker off (up) pull trigger, hold to the rear and cycle slide again, slowly release trigger to reset, take up any slack and pull trigger again(hammer should've remained cocked after cycling and then fall after reseting trigger and pulling)(it's been 15 years since I seen one, it should have a magazine disconnect safety––mag needs to be in the weapon to work)

cock weapon and on the left side there should be a small dot(may be red) by the decocker. As you slowly lower the decocker to decock it, it should decock before the bottom portion of the lever hits the dot. If not, the sear release lever is worn and need replacing.
If you can take it apart, check the trigger bar on the sides(inside mag well at top and the rear of the trigger bar. If it cracks, more than likely it's at the back. It will still work with it cracked, just will have a real gritty feel and you will even hear it.

Other than that, just the normal stuff you look over-check the muzzle good as well.

Hope that helps!

Mark