>>it is a marketplace-as-a-service
>To the extent this makes a difference
It does, because you're arguing as if Amazon sells stuff. It doesn't (just) do that: it's a market-maker that provides a venue for other vendors to sell stuff.
So it's already an error to talk about Amazon as a monopolist, when Amazon isn't the only marketplace out there with a global reach. But you're also making a category error, because you wrote:
>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.
Amazon doesn't set prices per se, at least not for vendors in its marketplace that are not itself. So it doesn't do whatever you think it's doing when you were writing that paragraph. Certainly there are "other competitors offering the same goods at different prices to compare" with, because Amazon's marketplace include other vendors. But Amazon is running a marketplace, not (just) being a vendor. There's a difference in category there.
>To the extent this makes a difference
It does, because you're arguing as if Amazon sells stuff. It doesn't (just) do that: it's a market-maker that provides a venue for other vendors to sell stuff.
So it's already an error to talk about Amazon as a monopolist, when Amazon isn't the only marketplace out there with a global reach. But you're also making a category error, because you wrote:
>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.
Amazon doesn't set prices per se, at least not for vendors in its marketplace that are not itself. So it doesn't do whatever you think it's doing when you were writing that paragraph. Certainly there are "other competitors offering the same goods at different prices to compare" with, because Amazon's marketplace include other vendors. But Amazon is running a marketplace, not (just) being a vendor. There's a difference in category there.