The best thing to do is look at Craigs List and Ebay and see what used ones are going for. Another thing to check is what Apple is selling that model for refurbished as a kind of cap on the price.
As always, no matter what the topic, look at comps. Several places to check are mentioned above and they're suggested in every "How much should I sell X for?" thread.
Well, I own a Wi-Fi only iPad2 which is working fine but might consider purchasing the next generation device when released, especially w/ iOS7 coming out soon in a non-beta edition; so, I might be in the market to sell?
Thus, some additional questions: 1) Erasing your apps/data - assume that a 'factory reset' would do the trick? 2) Warranty transfer, if applicable - possible? 3) For those w/ a cellular iPad - assume that the 'new' owner buys there own SIM card? and 4) Registering the new iPad & restoring from a computer - difficult or not?
I'm sure questions are additional concerns of those upgrading to a newer iPad - thanks for comments & suggestions - Dave