Also see Camelcamelcamel (http://uk.camelcamelcamel.com/) - they do a great job of price tracking on Amazon. They have historical data for a surprising number of products too.
We do this at my company, https://trackif.com. More than products (for instance we'll track rubygems, npmjs & github). I just haven't gotten off my butt to write a blog post about it since I figured "price tracking" wasn't a topic that'd interest the HN community.
Reminds me of stocks. How do you tell if the price is at lowest? If it is not at it's lowest you are losing money. If it is at lowest and you didn't buy it you lose money too.
IMO, "price is at lowest" doesn't make sense at all. If I'm are interested in buying something, waiting for a while to save a few bucks would give me more pleasure than knowing what the lowest price of this product ever was.
I have found prices varying a lot between sellers in flipkart. And flipkart most of time shows its own seller(WS Retail)'s price. So such a feature will be nice.
I read a Quora post by a Flipkart employee who said that the seller chosen on the home page is often the most trusted seller. I doubt if people would buy from sellers with lower ratings. Nonetheless, a feature to consider all sellers before notifying prices would be nice.