> So if today its Amazon and tomorrow someone else, why be up in arms about it

Because it doesn't matter who ultimately made an anti-competitive world marketplace, it's still anti-competitive and ultimately impossible to run morally when you participate in it in parallel. eg if ebay was undercutting other people's auctions via buyout prices under their own brand

But doesn't the customer benefit in that scenario? They get the items they want for a lower price from a brand they trust (they're on their website for a reason). And if the customer is benefitting, isn't that the best outcome of capitalism?

In that scenerio, and captialism in general, eBay doesn't owe sellers on its marketplace the right to sell items at all. They've just been given a chance to make hay while the sun shines.

Benefits are not static. What's good in the short term, doesn't necessarily mean it's good long term. Following the capitalistic ethos, once you have less competition, you can raise your price to match what the customer can afford (which is not competitive, it's exploitative) as opposed to what the market will bear (because there is no market, just a limited or singular purveyor). This is economics 101 stuff. China survives this by government control and investment in the monopolistic companies and ignores IP, allowing small companies to have opportunity to make knock-offs and creating limited theater attrition. When these companies start being noticed (either profit impacts or potential efficiency improvement opportunities, whatever), they are either purchased into the fold or forced out of business by governmental mandate. The US is not unique, but it's also not as free, for reasons that people don't point out very often...ie the draconian enforcement of IP is multi-edged.

I agree with everything you say. However, everything you mentioned about how Amazon could act in the future is speculation. I agree that when / if they start raising prices they should be considered a monopoly and heavily scrutinized and possibly regulated. But at this point, there is no justification in doing that as they haven't crossed that line.

I feel like a lot of people jump to attack Amazon on the basis of what they could do (because they're so big) instead of what they're doing currently.

> So if today its Amazon and tomorrow someone else, why be up in arms about it?

Because - I believe - people are overwhelmingly reactionary, easy to excite and short-term focused. Microsoft was the end of the world. Google is the end of the world. IBM before them. Yesterday Walmart was the destroyer of worlds that was going to kill every small business in every town in America. Countless articles were written for a decade espousing that as fact, inevitable, unstoppable. They were pure evil, everyone knew it. Nobody could compete with them. That reactionary emotionalism against The Corporate Overlords, is now retrained on Amazon as the destroyer of worlds, the evil entity that will crush everyone. Tomorrow it will be someone else.

The most interesting story about Amazon - which is being almost universally ignored - is that the growth in their retail business has already mostly stopped (some shareholders are finally noticing). In the next few years that growth will continue to trend toward between zero and low single digits. The mighty retail conquerer of worlds is running out of retail growth and mostly that is being ignored because it doesn't bolster the story of the moment that Amazon is going to eat everything.