For example a few years ago in Australia it was a big issue that the supermarket duopoly was intentially making their brands of milk too cheap. There was a kind of spiral where expensive brands weren't selling well, so farmers had to sign restrictive contracts with the big supermarkets, but the rates were too low to be profitable or even viable.
The conflict of interest sort of "broke" the free market for milk. I'm not sure if retailers should be forced to be "brokers or producers but not both" but I am sure that if they were purely "brokers" then we wouldn't have experienced this particular problem. (The fact that the supermaket business is so centralised is another related issue).
But in this summary [1] of the report that Australia's competition regulator released in 2018 we can see that the price/liter that farmers were getting during this period doesn't seem to have been affected by the $1/liter pricing.
The ACCC found (quoting [1]): "no direct relationship between retail private label milk prices and farmgate prices", and that neither the rate at which farms closed down, or their profitability was affected by the $1/liter milk.
1. https://www.farminstitute.org.au/briefing-dairy-regulation-a...
2. https://www.accc.gov.au/about-us/publications/dairy-inquiry-...
Amazon can use the same data they've been using all this time, but they must not put themselves in a special position in acquiring that data. If they want to publish their sales data for free or for a price that they themselves pay, that's fine, as long as others can also get the data.
Colluding to increase the base rate would be a kind of price fixing (i.e. illegal) and having the third party increase the margin wouldn't benefit Amazon at all.
EDIT: remember if the data is actually valuable, then Amazon could sell it for MORE than it costs to buy it back, since the third party can recoup its costs from multiple purchasers. (So the market mechnism helps make the data more accessible and broadly useful.)