You'll have to unregister before you can reset to factory settings, so the new user won't have any access to your account.
The B&N extended service plan is transferable, but it has to be transferred in writing by the seller (you). The service plan can be found at http://www.barnesandnoble.com/u/nook-protection-plan-terms-and-conditions/379002486/ and the address where you need to send the transfer information can be found in section 12.
The standard warranty is not transferable.
I broke down and bought myself a Nook Color, so I don't need my old 3G + Wifi Nook. Should I sell on Ebay or can I trade it in to Barnesandnoble?
well I bought one too many Nook 3gs for xmas gifts for my kids. I bought them for 99 dollars each on ebay from b&N and resold the one I didn't need on ebay. I started the bids at 9.99 and it sold for 157.50.
I just checked on ebay and they are doing better than 67.00. I would go to ebay, do a search and compare what is selling for how much (meaning nook 3g or nook wifi plus extras or not) to what you would be selling.