Instead, we are left wondering if Coinbase is selectively cancelling transactions that they can make a profit off of (due to the volatility of bitcoins)
I don't think the average HN user appreciates the risk environment payments companies work in. Imagine if a criminal gang organized enough to have a payroll had ten copies of people who were every bit as good at app security / financial risk management as e.g. Thomas, with two of them whose only marching orders were "Probe payments sites every day, quietly, and tell the team when you find one which is weaker than it could be. We will then conduct several months of quiet R&D on a software product which is more elaborate than that shipped by most YC startups. Our 'launch' will be deployment of that software product against our adversaries at the payments company, and it will blitz them so fast that by the time anyone thinks to pull the plug we'll be millionaires. Then we'll celebrate our good fortune until we do it again next week, to them or another company as circumstances dictate."
(Risk management, which a lot of startups need, is a high-leverage and very fun environment to work in. At present a lot of companies, large and small, custom build systems out of bubblegum and duct tape. If you get good at this you'll have a very rewarding career ahead of you, for many definitions of the word rewarding. I used to want to go into it but got sidetracked by the unexpectedly high amounts of leverage you can get using software to sell software.)
For a given time period
* Mark transactions as 'high risk' at random
* Prefer transactions which buy coins from the user at low prices and sell coins to the user at high prices
* Randomly cancel a subset of non-preferred transactions after you've seen the market move over your time period
* Profit
If you do this right, canceling transactions is like having a price oracle, you can profit with 'future knowledge' by leaving some transactions open for extra seconds/minutes/hours/days.