I like the TI resale program. I know it would have been very easy to get my TI89 off me after college for a steal. The difficulty comes from the fact that anyone who doesn't need that calculator anymore is almost certainly no longer around people who do. I didn't have the foresight to pre-sell it to a freshman prior to graduating. Good money says almost no one does. Great idea!

[edit] This could also spin into a rental program.