[dragonwriter]

> So can I define Epic having a monopoly on being able to buy things in Fortnite?

If price increases for Fortnite IAP don't, empirically, drive people to stop buying them in favor of alternatives of some kind, yes. Or, rather, you can't define the market that way, but if there is no substitution effect a US court considering an anti-trust claim is likely to see that as the relevant market, not some broader, say, battle royale game IAP market in which price-driven substitution does not occur.