But it would be victimizing yourself to not make the trade, and instead tell the other person your inside knowledge.
And it's not dishonest, because in a market, there is no implicit assumption that the buyer doesn't know something the seller knows, on the part of either one.
This is especially obvious in the stock market. If I think a stock is a sell at $X and you think it's a buy, we trade, but clearly one person knows (or thinks they know) something that the other doesn't.
Regarding ticket scalping: People selling tickets know that that's going on. If they don't want it, they will forbid you from doing it, and if you do it, it's breach of contract and subject to legal penalties. If not, go for it. There is no reason to sacrifice your own interests so that people who didn't get in on the sale earier and can't afford scalped prices can have their interests satisfied instead. In other words, don't victimize yourself.
Regarding HFT: If you are market-making, you buy at the best price (giving someone a good deal) and sell at the lowest price (giving someone a good deal). Yes, these people could have matched against each other if they wanted to play the game of predicting which way the market is moving and waiting a few microseconds, but you can also lose at that game, so institutional investors don't want to play it. There are no victims here. I assume this kind of thing generalizes to other forms of HFT, except when people/exchanges actually do cheat (exchanges: people don't let people cheat, that is basically your sole purpose for existing).
> The moral of the story: It's difficult for me, at times, to discern when a "cool hack" becomes "criminal theft."
You only victimize someone (other than yourself) if you commit breach of contract (implicit or explicit; fraud is a form of breach of contract) or actually use physical force. Otherwise, you aren't.
tl;dr moral of the story: don't victimize anyone (yourself, or others)