Many retailers require a buy back so that their purchase is risk-free as well. Also the 30% is not pure profit. There is some cost to Apple for the services that the Apple store provides. Further, there is no cost to the seller. Apple's model is a retail consignment model; there is no cost to the seller until a product is sold. In a grocery store, shelf space needs to be paid for. IMO the 15-30% cut is quite reasonable for the marketplace and support that Apple provides.The retail clothing market is not analogous to the App Store. I’m sure one could put together some tenuous comparison, but Apple is not risking money by hosting an app in the App Store. There is no up front purchase of X amount of units of an app with the chance that none of them will sell. Apple’s end is pure profit, no risk.
And think about this, Uber works on a commission model as well and is still not profitable.