[Cronos]

A retailer that sells 30% of Brand X's total output has control over brand X. They can decide not to sell brand X but sell Brand Y. This is the control they have, control over the seller, not the buyer.

[discostu]

But even then their power is limited by brand loyalty. If they chose not to sell brand X but the people that use brand X don't want to switch to brand Y then they just hurt themselves.