Always look for the words "Ships from and sold by Amazon.com." under the green "In Stock" (just below the price).
If it doesn't say that, then it is listing the next cheapest available dealer.

Almost always, Amazon is the cheapest (they seem to adjust prices to compete), so by default should be the main listing if they have it in stock... but very very rarely, a third party might be clearing out something cheaper, and Amazon's product/price is likely to be in the "other sellers" tab.