Those sales were never yours to lose, right?
Let's say I own a restaurant and instead of improving my food or service I go to my competitor's food suppliers and offer them extra money to not sell to any of my competitors. In isolation, there shouldn't be anything wrong with me making an agreement with another private corporation, but the effect of this is that it creates an unfair playing field for my competition.
Amazon's position as both the marketplace and a competitor in that marketplace, creates very similar "unfair" circumstances that benefit them uniquely on that marketplace (for example placing their offerings higher up in search results, or creating special "recommendations" specifically for their offerings, etc) not to mention, the unique access to information that they have into how well their competitors do (which is not inherently unfair, but when combined with all of the other aspects does make a decent case for fitting the "unreasonably constraining competition" section of anti-trust law)
Personally I don't like what Amazon is doing. But I don't think it is illegal under current anti-trust laws. The laws really need to change, but I don't see that happening.
I think the justification for why something like exclusive dealing agreements in monopolistic conditions is unfair could definitely be applied to why being a competitor in your own marketplace, while leveraging your customer's data, and boosting your own listings could be deemed unfair and lead to monopolistic conditions too.
Is that what Amazon is doing?
I get maybe for the older population but that’d put a 10-20 year cap on Amazon dominance before the majority of their customer base understands the internet enough to look for alternatives if amazons service isn’t good enough.
Now imagine a company that has been just as effective as Facebook at replacing/consuming/killing it's competition in the retail space and you've got Amazon. If Amazon went down for a proportional extended period, say for a few days, you'd likely see a massive spike in sales at other retailers just as we saw a massive spike in new site traffic when Facebook was down.