I saw on MobileRead Networks (a great site BTW where I often find the earliest reports of new mobile content) that Yahoo has launched a beta Mobile Shopping site. You can read what Yahoo has to say about it here. I tried the url Yahoo gave, http://shopping.yahoo.com and got 404 (Not Found) on my phones. I then tried on the Openwave, Netfront and Sony-Ericsson WAP emulators, which all support WAP2 which is required according to Yahoo. Only the Netfront emulator was able to load the page. The problem seems to be that Yahoo is using the same url for both the desktop and mobile sites and then using browser detection to redirect mobile users to http://acid1.oa.yahoo.com/mbl/shop. For some reason the redirection, which is unevenly supported by WAP browsers, was failing. A lot of sites do this sort of browser detection to use the same url for desktop and mobile sites or wap1 and wap2 sites. It's a nice idea in principle, adding ease of use but it always fails on some percentage of devices. I really wish sites using this technique would publish alternate urls where you can be sure of getting a specific type of content such as desktop, wml or xhtml-mp. Anyway, once I got to the site I found it simple and intuitive to use. You just enter what you are looking for i.e. "Unlocked Nokia 6200" and it returns a list of items for sale on various e-commerce sites and Ebay. It only gives you a short description, the price and the name (but not the url) of the site. There is no way to visit the seller's site or get more information. At this point it's just a price comparison tool - but it is a beta.
I used the YesWAP html to wml transcoder to create a WAP1 interface to Yahoo Mobile Shopping for those of you who don't have the latest phones. Point your WAP1 phone's browser to YesWAP.com (or wml.yeswap.com if you phone has a problem with my browser detection) and navigate to the shopping link.
Yahoo has some strong competition in this space. Google's Froogle (http://wml.froogle.com/) is very similar except it's a WAP1 site. Ebay has both WAP1 and WAP2 sites and you can actually bid using them. Amazon has a great site where you can search, read full descriptions and user reviews and make purchases. PriceGrabber is similar to Yahoo and Froogle but allows sorts results by price, gives retailer ratings and a link to make a phone call to purchase each item.
You can see my descriptions (I can't really call them reviews - yet) and ratings for these and several other shopping sites on the Wap Review Shopping page.