I recently came across a textbook aggregator that listed all sellers and their prices on the net. It saved me so much time and money from actually having to visit and compare each site individually.
I will do my best to go back and find if I bookmarked it or exists in my history. It included very fast comparisons and prices of all online book sources for new and used editions (not just textbooks) including Alibris, Amazon, BN, eBay, and tons more I had never heard of. The results were very accurate and up to date.
Here's an example (I do not recall if this is the site I used):
http://www.allbookstores.com/
In the search enter this:
essentials of health information management (just a book I saw in a listing)
Click the top
Compare Prices
And there you have it. All results, condition, price, with shipping, special offers. Blazingly fast site