So 30% of the eBay fees and 30% off the Amazon marketplace fees should be fair game
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%.
Are you sure / how? I’m not going to sign up for robinhood to confirm but my brokerage transacts with my bank, not in-app purchasing, pretty sure robinhood is the same. I’m pretty sure the differentiation is that stocks aren’t considered digital goods. And neither is coinbase’s mainline business which is allowed to facilitate buying & selling crypto without paying fees, even on their transaction fees, which users pay.