It's a similar problem to why we have antitrust laws. If this was you or I, there wouldn't be a problem. If you are the only / pretty much the only store used

It's a similar problem to why we have antitrust laws. If this was you or I, there wouldn't be a problem. If you are the only / pretty much the only store used then it (to me) would clearly be a problem for consumers. You'd be using your monopoly in one area (marketplace) to benefit you elsewhere (selling USB cables) rather than competing on a level playing field. In between it becomes trickier. At what point are they too big? How dominant would they need to be? I think there are differences to physical stores partly due to scale. If a shelf has 10 different variations of basically the same thing that's the equivalent of all products being on the front of the search results. If there are hundreds, then the ranking becomes vital.