You are moving from a bike that has last generation components to a bike that has many generations (depending on when released/bought) older components. While not necessarily a bad thing you have a bike that is current and good other than the sizing issue.
Disc brakes are a HUGE thing for these bike as there are pretty much no bikes (except very entry level) that do not come with disc brakes. Without pictures of both bikes of exact models with display of components we are not going to be able to make an accurate evaluation other than heresay.
While you would be moving from a HT to another HT, the cost to upgrade components (or in the most recent post build a bike) is going to be very cost prohibitive. I know that most of my LBS (local bike shops) will do somewhat of a trade-in if you have a decent relationship with them so that would be a better avenue than the trade you are looking at.
Finally, the hard thing with "BlueBook" is that it is an estimated value on a mint condition bike. Bikes depreciate pretty quickly once they are ridden and you have stated that the one you are looking at is not only 5yrs old but also has probably been ridden pretty hard in that time. You could get it only to find out that you have TONS of issues, think of it like buying a car that is 2ish yrs old only to trade it for a slightly better car that is 5ish yrs old with LOTS more miles and undocumented history. It is a
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EDIT: just went and looked on BikePedia and there is no 29er Stumpy offered in 2009 let alone as a HT and canti-brakes. Walk away very quickly as someone it trying to pull a quick one on you. Continue to sell your bike on CL or try the trade-in approach at your LBS.
