pure "mysimon" clone, with all bells and whistles, including backend administration interface.
users must be able to register, rate and add comments.
webmaster must be able to manage merchants and users, launch price harvesting.
merchants must be able to register, submit froogle-like products catalog
must be reusable, conform to Model-View-Controller design pattern, and display only Smart (human readable) URLS (with product name and category in URL, like many blogs around).
PHP5 is better. Static file generation would be a plus.