1- what venue is he selling them in? if it is trustworthy sale venue that you know...(ie like ebay) I would imagine he's the real deal. Not to mention ..if a scammer or thug wanted someone's money nowadays...he'd be trying to sell you a genuine "ipone"
He'd hardly be trying to ensare with a couple n8x0 devices.
Rest at ease there.

2- good price...If you had the money one of each is my suggestion...esp if it comes with batt . and all the gear.
Frankly ...even if it was just the device and stylus ...nothing else...it's still a great deal. If it were me...and he has a whack of them...I'd be buying at least 2 of each...that way you wouldn't feel nervous about bricking one...mind you....having a bunch is just a placebo to calm you...fact of the matter is...as long as you always read about what you want to fiddle with....and if it is some obscure thing or alt os that clearly states you could brick your device ...then don't be tempted to do that thing...pretty simple.
Frankly I have never bricked any of mine ...ever.....not saying it isn't possible...just always follow good advice and proper instructions and you'll not go wrong.

3- Instagram?...never played with it...not sure about whether it will work.
Ebooks?...oh yeah....quite a few ebook readers.fbreader is my fav....there is even a text to voice prog. so it can read them to you...
Video call? ...Skype was going to go down that road with the n8x0 but it never happened...so Skype has only voice and text.
Gtalk has it though...it works great between n8x0 devices ...I used it for a long time ...I believe it will work n8x0 to other devices ...haven't fiddled with it for a long time.

4- Most importantly ....you must not look at it as simply a gadget...or another internet tablet..or some kind of phone alternative...it is really a full computer...hell you can run a few distros on it..The stuff you can do is pretty cool. It is only limiting by your curiosity and intention.

To poorly coin the phrase and quote... the NIT (Nokia Internet Tablet)..."...Is the beautiful mistake the Gods made ...that man got his hands onto..."

REMEM. too ....the most dramatic reason why the device line of the n8x0 has little movement currently in devel or even in the forums is not because the device isn't good anymore...nor that people can't devel anything further...there is still alot of devel that could be done...vast amounts of new stuff...the fact is...many ...if not most in the community...moved on to the next platform available the moment it came out...n770-> n8x0->n900->whatever wanting more of whatever they were looking for...so it isn't like it just hit a brick wall because it is an old device or people couldn't do new stuff with it.....there is still alot of undiscovered country there in the n8x0 ...you could come up with the next great prog that brings more people back...perhaps a good backport...or a prog that people want on their n900's ...so really... movement in the n8x0 sections here is really about numbers...of users.