Alternatively: there's no trouble at all, Walmart/Safeway/etc are all clearly engaging in this anti-competitive practice and must be reigned in. Marketplaces need to be regulated as neutral grounds for sellers, the marketplace cannot double-dip and compete against sellers or engage in practices that reduce competition between marketplace businesses.
Generics and knock-offs are fine, it just needs to be done by independent sellers.
Practices like down-ranking sellers for offering better prices elsewhere is just blatantly violating any sense of neutrality, reducing competition among marketplaces and increasing prices for customers.