Greentoe is an interesting concept. You state the price you're willing to pay for an item, and they find you as an authorized seller who is willing to sell at that price. I have only tried this once, for the Tamron macro lens (newest version, that you can find for $649), and it worked flawlessly and easily. I Initially offered $550, then went to $570, which was accepted. No shipping, no hidden fees, it was indeed sold by an authorized reseller, I checked on the Tamron site. The seller included a retail receipt for warranty reasons.
It seems to me, and I have no affiliation with GreenToe, that this is a very good site for slickdealers, assuming GreenToe has the item you are looking for.