Book swapping online is a great, environmentally conscious way to build your library of parenting books or children's books. I participate in three book swapping sites actively, and in this way have supplied my kids with endless reading, and myself with an enviable collection of parenting resources.
Book swapping involves loading an inventory of books (or in some cases, CDs, DVDs or games) that you are willing to give away to someone who is looking for those items, and in trade you can get the items that you want. The details of how trades and credits work vary from site to site, but generally you pay to send out the items you are giving away, and others pay to send you the items you request. Each item will generally cost you $1-$3 to send depending on if it is light enough for first class mail, or if it is sent via media mail (a discounted post office rate for books and other qualifying items).
There are a couple sites that have fees involved in each trade – I don't begrudge the sites wanting to make a few cents, but there are enough out there that don't charge, that I haven't felt the need to turn to these. The main way the non-fee sites earn money is if you use their postage printing services. The sites contract through endicia.com or other service so that you can print shipping labels with the postage already included and keep a small free for each transaction. You also pay a little more for add-ons like delivery confirmation, but if you don't have a post office nearby, it is convenient. You can also stock stamps in your house and avoid the fees for smaller items; however, using the included postage services allows you to also bypass the national security restriction on sending items more than 13 oz. from home, since the postage is traceable.
I've found book swapping to be an invaluable tool in building a literature-rich house for my kids, and saved literally hundreds of dollars doing it. I've swapped for nearly the entire Magic Treehouse collection for my daughter, as well as books for kids about elections and holiday, cookbooks, craft books, science readers, early readers, board books, kids music and movies and much more. For myself, I swap for current fiction, an otherwise expensive luxury, DVDs and CDs which I would otherwise deny myself (it's often cheaper than a DVD rental, and I can keep it or pass it along), business and spirituality books, as well as my giant parenting resource library. I've used this to build and extensive reference library for my BellaOnline work and my planned future endeavors as a parent educator (sadly, I could not swap for the necessary bookshelves!).
The three main sites I use (linked below) are PaperBack Swap, Bookmooch and Swaptree. They are all slightly different. All three allow you to list your inventory and to have a wishlist of books you want that are not already available.
• Paperback Swap is currently free, has a large inventory and allows only books in good condition. Listed books and wishlist books are first in, first out (FIFO). So if you want a book that gets listed a lot, you will eventually get it, even if many are ahead of you (I recently got one where I started more than 250th on the wishlist). When your turn comes up, you get to accept or reject, or you can automatically accept. On the giving side, if you list a popular book and are willing to hold onto it for some time, you will eventually get to swap it. Sent books give you credits which you can redeem for when you want. You also get a 2 book bonus once you list your first 10 books. They have a pay-for-postage service.
• Bookmooch is free and allows books in a wider range of conditions, but has a place for optional condition notes which you can view before requesting. When your wishlist books are posted in the system you get an email, and the first one to claim it gets it. So if you are fast and get there first you can get a book you might have to wait a long time for on a FIFO site, but there are no guarantees. Bookmooch also allows Amazon alternate ISBNs (that is, when Amazon makes up an ISBN because an older book or odd version may not have one), so you may be able to find (and list) books here that you can't on other sites. You get credits when you send books that can be redeemed when you want. You also get fractional points just for listing books and for acknowledging received books. They do not have a pay-for-postage service, so you will need a postal center nearby to send items more than 13 oz., or join your own online postal service.
• Swaptree (now Swap.com) is a bit unique. First, you can do books, CDs, DVDs and games. Second, there are no credits. Swaptree compares what you want and what others want, and creates either two or three-way trades. Trades only go through if all those involved accept them, so you only send an item when you are getting an item (and vice-versa). If you have a popular book or item, you can choose from sometimes tens of thousands of other items. They have a pay-for-postage service. As of Oct. 2010, swap.com charges a small fee per trade and requires delivery confirmation for trades.
In addition to these three, I also been part of a few others but it was getting overwhelming managing inventory and wishlist on so many sites. It can take a bit of time to keep those lists up to date if you do more than one site, so if you are just trading casually, you may want to choose the one that works best for you and stick with that.
If you become a "serious trader," in addition to trading your own household books, I've had great luck obtaining books through my community freecycle, or inexpensively at garage/yard/rummage sales. I really like that I can give books a home with someone who really wants that item, and in return, get something I really want too. It keeps books out of trash and recycle bins, reduces consumerism through reuse of books, and saves money – win, win, win!
Here's the links to my three favorite book swapping sites:
Swap Your Used Books - PaperBackSwap.com