The things in my list were pretty passive things, as saving money goes. It doesn't take a lot of willpower once you've already made the call to reduce a bill.
The book went for $22. The reimbursement for shipping was slightly less than the real cost, plus there was a small commission for half dot com. I got a free, used box, just the right size, from my workplace. And now I'm about 20 bucks richer.
You're not going to fund your retirement by selling old textbooks and bestsellers on the internet. But if you need to free up some space you could try it out. As I go along, the things I can't sell I plan on donating either to the libary or the salvation army.