Simalar question here:
http://www.seomoz.org/q/competitor-scraped-ecommerce-product-overview
Short answer, e-commerce site copying products descriptions from other sites is common place, and there is little you can do about it, but I believed google will know that its your content as it will most likely be crawled on your site before your competitors have a chance to find, copy, post and then get crawled.
