It almost sounds like you are talking about a datafeed when you say you just pick a category. Here is a beginners article I wrote about datafeeds.
http://www.5staraffiliateprograms.com/datafeed1.html Is that what you mean???
If so then the software most affiliates use to build a product store from a merchant feed is called WebMerge.
But my other mod here Tele, has a script that converts a datafeed into Wordpress. If you are already familiar with Wordpress this may be a better option for you. Plus you can tell the software to add just few products a day or whatever so you always have fresh content.
Heres the info and a video that shows how it works.
http://www.wordpressdatafeedimport.com/affiliatefeed/