No. The only requirement is a "dominant" player. https://www.ftc.gov/enforcement/anticompetitive-practices

This is the reason why Microsoft and Google have both been investigated for anti-competitive behavior even though neither one of them was a monopoly.

The Sherman Act prohibits using a dominant position in one area as leverage to exclude competitors in the same or different area.

In other words, a dominant player has to play (more than) fair in any new markets.

I say "more than" because if Amazon was not so dominate in ecommerce this behavior would be perfectly fine.

> Sure, but you're allowed to be anti-competitive as long you're not in a monopoly position in the marketplace.

An antitrust "monopoly" is defined by market/pricing power. I wouldn't be all that confident that Amazon's online retail operation wouldn't be found to have such power.

You don't have to have a literal monopoly afaiui, I think leveraging a strong market position in one area to benefit a different market can still apply. Not a lawyer though.

In the US. I'm not sure that's true in the EU.