I want to write software for Amazon and eBay to sale.

I want to create a tool that I can sell. I know Amazon APIs very well, and eBay Finding API. Something like Product rank by UPC/ASIN or ID, UPC lookup, or Price alert. I am not sure what would be more useful for Amazon , eBay sellers and Affiliates. Please suggest me idea for the app. NOTE: I am not interested in hosting or subscription based Saas Application more. I want something to provide as download. And just web application using PHP and JavaScript. Thanks for your time.