To the extent this makes a difference, because of Amazon's efforts to monopolize various market segments by undercutting competition, it decreases the amount of information Amazon has about other market participants. A perfect monopolist, by setting monopoly prices, decreases the value of the price information it gets through transactions, because there are no other competitors offering the same goods at different prices to compare to, so the prices of the monopolist's transactions convey less information about the preferences of other market participants. That makes the market less efficient.
To the extent this makes a difference, because of Amazon's efforts to monopolize various market segments by undercutting competition, it decreases the amount of information Amazon has about other market participants. A perfect monopolist, by setting monopoly prices, decreases the value of the price information it gets through transactions, because there are no other competitors offering the same goods at different prices to compare to, so the prices of the monopolist's transactions convey less information about the preferences of other market participants. That makes the market less efficient.