Many ecommerce sites have session IDs or user IDs in the URL of their pages. This tends to cause either the pages to not get indexed by search engines like Google, or to cause the pages to get included many times over and over, clogging up the index with duplicates (this phenonemon is called a “spider trap”). Furthermore, having all these duplicates in the index causes the site’s importance score, known as PageRank, to be spread out across all these duplicates (this phenonemon is called “PageRank dilution”).
Ironically, Googlebot regularly gets caught in a spider trap while spidering one of its own sites – the Google Store  (where they sell branded caps, shirts, umbrellas, etc.). The URLs of the store are not very search engine friendly: they and are overly complex, and include session IDs. This has resulted in 3,440  duplicate copies of the Accessories page and 3,420  copies of the Office page, for example.
If you have a dynamic, database-driven website and you want to avoid your own site becoming a spider trap, you’ll need to keep your URLs simple. Try to avoid having any ?, &, or = characters in the URLs. And try to keep the number of “parameters” to a minimum. With URLs and search engine friendliness, less is more.