I think what you're seeing here is the fact that nobody wants a 8gb version, and I'm one of em. But hardware wise, the only difference is the 8gb part, so it's not "obsolete". That being said, you should still look at it that way for buying purposes. Personally on cragslist, I like to low ball an offer with a "and I can pick it up today". Even though you've already set up a meet you can still low ball him. Either go to a staples, or say you did, and tell him that you just can't pay that much for a used damaged one when you can get it new for X amount. I myself wouldn't pay $100 for it, but if I were in your situation, I would probably go $130. But whatever you end up paying, so long as it works, he'll be happy with it....
Speaking of it working, check the speakers! Play a song wide open and listen for any cracking for the speakers. That's a minimum $35 on ebay IF you can find them. Be sure you're near a wifi access point to check the wifi (mcdonalds comes to mind). Speaking of going by staples, you should just to get a feel of what it looks and feels like so you'll know the difference (if any)....
Good luck and happy Bday to your kid....