That doesn't seem to hold up. If I buy something on eBay it's not any different from, e.g., buying stock on Robin Hood. There is an agreement between two parties, neither of whom are the people distributing the app, to make an exchange of money to [the eBay item] or stocks.

Apple does not claim a cut of the exchange of stocks, but they do claim a cut of the transaction fees for the exchange of stocks. If eBay or Amazon charged a transaction fee to facilitate their transactions, that would be an in-app purchase of which Apple would claim 30%.