I finally got around to documenting and making available my Amazon wishlist tool. If you maintain a wishlist and want to periodically see if any items on it are available for cheap you can put this in your cron:
0 * * * *
/usr/local/bin/wishlist --token=MYTOKENID --wishlist=MYWISHLISTID --firstname.lastname@example.org
Which means at the top of every hour wishlist will go and look at my wishlist and email me if any items on it are available for cheap: cheap being less than 50% of the coverprice and/or less than $5. Cheapness is configurable. It uses Amazon's Web Services, so you'll need to get a token (free). I imagine that there's some CPAN module that does this, but there wasn't at the time when I wrote this about a year ago (I don't think).
If you use it you'll get lots of hits from used bookstores around the country. But I don't mind used when the quality is good, and I've been really happy with the quality measures on Amazon.