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I haven't seen one in a while, but this sort of thing used to go off the rails on amazon. I tried to buy a 10 trillion dollar chair--presumably algorithmically priced--but the checkout process broke before I could see what the CC company would say about it.

Looking at supermarket (or any) prices, publicly posted, in-store or online, is not remotely grey market.

Scraping them with high fidelity and frequency, and then selling that data commercially... and then buying that data from a provider and updating your prices using them...

Probably isn't something a company with potential monopoly concerns (read: Walmart et al.) would want be be overly open about.

> Why collude when you can just hire grey-market high frequency scrapers to detect your competitors' repricing and trigger your own?

What the thread actually said is this:

> My guess was: tacit collusion, meaning, oligopolic price coordination without explicit coordination.

Which is what you described.