To the extent it's a competitive market (and it seems like it would be-- a lot of potential buyers and sellers), the price should be determined by the minimum average cost, not by the willingness/ability of the buyer to pay. This outcome is not dependent on the ability of the buyer to walk away (as you note, their life might depend on it so some of them can't), but on the sellers pursuing their self interest which we can probably count on them to do.
It may turn out to be a competitive market, I just don't think it should be. In a truly competitive market Peter Thiel could just outbid a sick person for a particularly choice pint. In this market system whether a person gets blood or not will depend on their ability to pay for it not their need of it.
