In an enormous number of cases Amazon both sells products and the same exact product from a different supplier, and they sold the product before opening up the store to others. That's the definition of a competitor.

The fact that they didn't sell every single product on planet earth doesn't mean they aren't a competitor. They are practically in the market for "easy to manufacture by a third party in china consumer products" market. If a company adds so little value that Amazon's 5th rate people (The best minds at Amazon aren't sitting up at night worrying about the guitar pedal looper market) can beat them, they aren't adding value. And, one single distribution channel (the amazon website) doesn't define a market (with almost zero exceptions, amazon's website not being one).