"Types of agreements that have been held per se illegal include agreements among competitors to fix prices or output, rig bids, or share or divide markets by allocating customers, suppliers, territories, or lines of commerce. The courts conclusively presume such agreements, once identified, to be illegal, without inquiring into their claimed business purposes, anticompetitive harms, procompetitive benefits, or overall competitive effects."

Obviously small players aren't worth prosecuting but that's a different question.

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.

Collusion is always illegal when it's anti-competitive, whatever the size of the participants of the collusion. The key point to take away though, is that the authorities are unlikely to enforce antitrust laws against minor/negligible market participants. However, as little as 2% combined total market share has been enough to justify enforcement of antitrust laws for collusion.