The challenge is to get rid of some set of books that have accumulated, while simultaneously fulfilling the need to always have some kind of new reading in the house.
A new (to me) way to do this is Paperback Swap, a system which works as a trading board for books. You are entitled to books in the proportion that you send books to others, and every book you request is free to you. The catch is that you pay postage for the books you send to others.
Because of the sender-pays structure, many of the books listed are lightweight and cheap to mail. There's some calculus in people's minds that probably says "I'm willing to pay the cost of shipping to get rid of this."
I can imagine a lot of other things that could work the same way, looking especially for items that have relatively high value compared to weight, but where you might accumulate unwanted and unused items over time that have more swap value than use value. It looks like Swap-bot fills some of that need for Etsy-class homemade items.
More on Discardia from Dinah Sanders.