Theres an addon that I've been using for some time that gives you the best deal on whatever you're searching for. For example if you're on ebay looking at a fork fox it will just pop up at the top of the page and tell you "item is listed for _____, its $xx cheaper here, here and here.." It's called InvisibleHand, and is for firefox and chrome. I'm not associated with it by the way...
Hope it helps
EDIT: I just had a look at the main site and it seems that it's available for all browsers:
P.S Don't expect it to work right away, once you're on a page it has to search other sites which takes a few seconds.
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