Emily's Cat
Rarely prone to hissy-fits
If the large buyer completely dominates the market, then it isn't a competitive market. It will distort cost-benefit feedback from the end consumer. In particular, if the government is involved, you don't get cost-benefit feedback from the end consumer at all. What you get instead is politics, which tends to favor those who are the noisiest or most well-connected.
Just for consideration, I think that a lot of medicine cannot be a free market ever. It's a monopoly on the part or the manufacturer. In many cases, those manufacturers exploit tacit coercion in order to command a significant portion of the buyer's surplus from the negotiation.
If the choice is between bankrupting oneself to afford the drug and getting to stay alive or not getting the drug and dying... I'm not really sure that's a valid exercise of agency. Kind of like if you have a choice of killing your dog while a gun is pointed at your head and staying alive, or letting your dog live and being shot... that's not really a "choice" to kill your dog, is it?