I guess the answer depends on the condition of the unit. But note that they no longer make the 139, which (to me) makes it more valuable due to scarcity.
We bought a 2012 new, and it cost us about $15,000. But we also paid for some modifications, like leaf springs and a flipped axle.
Did you put the rear bumper and hitch on yourself? That is valuable, if it is well secured to the frame. Most units of that size can't take a rear bike rack, and yours can!
I would also think that the value would be affected by your location -- if you are in a well populated area, there will be greater demand.
My wild guess -- $10,000?