For trades, Amy had once put up a site link to:
www.thebookcart.com
This site allows you to set up an account with a $10 balance and list your own books for trade, or select books others have listed for trade.
Every time you mail a book out via media mail, you get a $1.75 credit to your account. Every time you select a book to receive, you get debited $2.25.
The $.50 difference covers the site's administrative charges.
They only handle paperbacks. When I've mailed books out, I usually use my husband's cushioned mailers he's bought from uline.com for Ebay--cost about $.25 I also don't mail via media mail when first class is cheaper. I've never not had my expenses covered by the $1.75 credit.
You won't find every book you're looking for there, and I do wish that you could sort "mysteries" for example by author as well as title, but it's a pretty good site.
When purchasing other books, I usually put the title into shopping.com and purchase it wherever the price is the lowest (and if overstock has the item, it's usually at overstock.)