Amazon Search
http://wordpress.org/extend/plugins/amazon-search/
Incorporate that. so when you search the results are displayed below the classified results in a box labelled Amazon Results

Anyone with an amazon affiliate account will understand where I'm going with this
*cough* 15% profit per sale *cough*
Im working on doing my own incorporating of Amazon Results and like the Facebook incorporation ill tell you how it goes
[url=http://MyUsedText.com]Buy and sell used textbooks![/url]