[zngtk4]

At what point does a casual conversation become illegal collusion?

Unfortunately, the law provides no clear definition, just as there's no clear point at which competitive behavior become "anti-competitive."

I'm not sure I'd agree with the statement that it's "VERY hard to make an anti-trust case stick" -- the antitrust laws are used a lot more than you hear about in the regular news (and the government doesn't have to be the one who brings them since you can be sued under the laws). For instance, in 2003 Nestle and Dreyer's Ice Cream were blocked from merging on antitrust grounds because the government defined the market they would gain a large share in as "The Market for Superpremium Ice Cream" (not the market for desserts or the market for ice cream). Many other deals aren't even attempted for fear of antitrust.

Not only that, but you can be found guilty of price fixing even if you didn't have a conversation about prices.

But if you're big enough for this to be a concern, you're already thinking about hiring Washington lobbyists to protect yourself and not reading Hacker News.