The point is that we should be clear about what exactly the issue is. "The sellers are telling each other the prices they accepted" is not the issue, because t

The point is that we should be clear about what exactly the issue is. "The sellers are telling each other the prices they accepted" is not the issue, because this is allowed and generally considered conducive to efficient markets (even if pricing info in that market has traditionally been treated as proprietary). There's at least three distinct additional features one could point to, and I'm not sure which (or the conjunction?) is supposed to be crucial: (1) That sellers are exchanging pricing information among each other without also revealing that info to buyers. (2) That the shared pricing information is unusually comprehensive. (3) That sellers are tacitly coordinating through the algorithmically suggested prices. As far as I can tell, the legal issue is #3, you are (I think) suggesting the important feature is #2, and my tentative opinion is that #1 is probably the most influential in distorting the market to shift surplus from buyers to sellers.