Honestly the value of them seems to ebb and flow in value with how many are available at that particular moment ....as well I have noticed that whenever someone writes an article lauding the n800 ...EVERYONE has to try to get one then.... the fickle mob eh

My best suggestion would be to wait until there is a real lull in listings on ebay...say a few (3 to 6) months ...and don't bother trying to sell over Christmas or after for a while...Sell them individually and you "may" just luck out...otherwise...25-75 a piece....tops...sans shipping costs will probably be your profit....I personally have enough n800's but I keep a close eye on how much the n770/800/810/900 go for depending on the time of year...available auctions....etc. That would be my best guess.
Way back in the day I knew nothing about the nokia tablets...let alone linux...2007 came ...got one and started lurnin
Tech lifespan-wise that is forever...but that is also based on "disposable" garbage tech in my opinion...the n800 when it came out wasn't cheap....bloody expensive....for now.... most people want exploding graphics and such rather than truly cool ability in a device.
you could do worse than keep them...remote desktop pc manipulation via wifi regardless of distance...stream your pc media...skype gtalk...vid-chat...multiple operating systems...heck ...universal home remote....list goes on....it still is a great device to truly learn on and really do stuff with.

There aren't too many devices historically that the manufacturers allowed and encouraged people to experiment. Mostly devices "seem" really cool nowadays...but the corps. have locked down the devices so you have less options and must use what they provide...and/or there are increasingly obvious private and personal security issues where people don't know if the phone makers aren't spying on their own consumers anymore even...let alone psychos, terrorists, malevolent or benevolent governments. For my own peace of mind I like to have some control over the devices I "own"....however minimal that control may be....

Bonus for you is.... you have a 2nd one as a back-up in case you brick the 1st....playing around with it.

Anyway...want to get rid of them quick..possibly the buy and sell of the forums here may help....good luck whatever you decide.