I guess, we should not believe google anymore.. I had built this little website that uses google's shopping api - <a href="http://www.shoop.in/" rel="nofollow">http://www.shoop.in/</a> .. All effort wasted, I should have seen what was coming..
Similar to InvisibleHand is a browser extension I built called PriceAdvance (<a href="http://www.priceadvance.com" rel="nofollow">http://www.priceadvance.com</a>). We have a browser addon for FF and chrome which does realtime price comparison of the products you're browsing and offers coupons.
We expected this. We were building our product catalog over time and wrote our own elasticsearch based search.<p>The prices are updated daily and we don't do product availability.