For Secondhand Books in General

• The Advanced Book Exchange-- The ABE is a searchable, frequently-updated, central database in which affiliated bookstore owners from many places list their inventories. You look for what you want, and the ABE tells you who has it and how to contact them directly. It's also possible to order your book from certain booksellers using the ABE as middleman (you enter credit card data over the secure server, and ABE does the rest), for a small fee.
  >> Note: I have found this site more useful than all others put together. Often I'll learn about the existence of a cricket book at one of the sites specializing in them, then come here to find a cheaper copy from a general bookshop.
• Alibris-- A posh version of the ABE. This site, however, not only puts more of an emphasis on rare books, but also insists on being the middleman in all transactions. Upshot: things just about always end up costing you more.
• Bibliofind-- And this one used to be another ABE, till Amazon.com bought it out. Now it's a search engine for the Amazon (US) Marketplace and Z-Shops only.

For Cricket Books in Particular

• Ian Dyer Cricket Books-- an online shop with nothing but cricket books, scorecards, etc.
  >> New-- you can now order directly from this site using a credit card and their secure server.
• Boundary Books-- another online antiquarian cricket book dealer.
• J.W. McKenzie-- and another nice one.
• BodyLine Books-- and another.
• Bowmore-- and still another. :-)

You Can Always Try Here, Of Course

• Amazon.com &
• Amazon.co.uk--
  
  The places to look for books in print, they also offer secondhand books through their Marketplace and Z-Shops. When available, used copies are offered on the same page as the new ones (at least at the US site).

Finally, Some Auction Sites

• eBay auctions-- Just search; there are usually hundreds of cricket-related items going at anyone time, and some of these are books. I've picked up a few that way.
• Yahoo auctions-- It has nowhere near the amount of stuff that's listed at eBay, but I found a book I wanted here, and had no competition for it.