This conversation has been going on for a little while already. Elizabeth Warren directly addresses proprietary markets (i.e. participating in you own) in her plan to break up big tech companies[0]. Her argument essentially boils down to either participating in someone else's marketplace to sell goods, or running your own marketplace without being able to transact within it, but preventing the ability to run and participate within a single market.
0: https://medium.com/@teamwarren/heres-how-we-can-break-up-big...