He said that Amazon is setting the wholesale rate, similar to what Walmart does. There is no evidence of that. If there was, earnings, or lack of, would certainly not be such an issue (Amazon reported less than expected EPS last quarter). As someone else said above, their earnings do not support the share price.
I would also add that, unlike Walmart, there are thousands of sellers using Amazon's portal/website. For almost every item, there are alternative sellers you can choose from. Amazon doesn't force you to purchase from them.
You're not familiar with selling on Amazon. I can't say for all accounts, as there are different levels and different associated features (such as richer product display abilities with higher end accounts etc), but they take x% off your retail price, that is what they pay, non-negotiable period. The x% varies by product category.