Markets can’t see the product quality of a monopoly. It won’t be reflected in the metrics because there’s no competition to anchor the earnings to the real consumer value. But that doesn’t mean quality isn’t a factor- it makes them vulnerable to disruption.
Warren Buffett is known to trade on product quality (he buys what he uses). So his sale could be based on that.
Amazon isn't a monopoly, it has 8% of retail in the US.
There's no real evidence that this trade was made by Buffet himself, and it's part of a general major sell-off of Amazon that transparently did not temporally align with the idea that Amazon's retail business has suffered a decline in quality.
This is the market making a (reasonable!) judgment that it lacks confidence that Amazon's capital expenditures will pay off.
Warren Buffett is known to trade on product quality (he buys what he uses). So his sale could be based on that.