>there is no such thing as an objective "universal good"
I guess here is where we part ways. I can understand it in the sense of objective morality not existing, but I don't think it makes sense to say there isn't a common good. I don't know what the word good means if it doesn't refer to something that happens between people.
I think it's a basic function of groups that they establish what is and isn't good - that forms customs, then law. Obviously, this is a flawed process, but I don't think it makes sense to go total nihilist about it, and just say because you can't prove positive and negative outcomes about given actions, you shouldn't try.
We're fully capable as a society of discussing outcomes of various actions and laws - and that's what we've been doing since the beginning of time. You're doing it yourself when you say 'actions that harm non-participants should be out of bounds'. You're just drawing arbitrary boundaries around the few rules anarcho-capitalists consider sacrosanct[1] and saying that these are obvious, while others are matters of personal opinion that are indeterminate.
I think the idea of 'voluntary trades' is also a bit strange. All trades have side-effects, and most trades are with an element of coercion, natural or otherwise - people need food and shelter, they can't simply not trade for them. So even if you freely trade to build a dogfood factory next to my house, I'm implicitly part of that trade, because I live next door (or if you push the argument along, we're all implicitly involved in basically every trade, because we live on the same planet - e.g. global warming).
[1] - by the by, but I'm pretty sure these rules wouldn't be enough. If you look at early modern history, you can see that a lot of state formation was driven by the needs of capitalism - as states respond to one crisis after another.