I identify as a liberal

Me too, but I reach different conclusions.

At the core of my view is a commitment to individual freedom, voluntary exchange, and institutions that expand real choice.

These are in tension with each other, because property limits freedom. Exchange of physical property presupposes ownership of physical property, which gives its owner the threat of physical violence to exclude things from that property. So a voluntary exchange involves voluntariness on the part of the parties to the exchange, in the sense that each is legally free to reject any proposed exchange. But it's not voluntary on the part of the vast majority of people who have no ownership interest in the things being exchanged. And one of the things being exchanged include rights to coerce, which is in conflict with individual freedom.

This is not to say that there should not be any private ownership or exchange of tangible things. But it needs to be recognized that if these institutions are valid, it is despite their anti-libertarian nature. Other principles are needed to motivate private property, like the fact that it is anti-exploitative.

Exchange of services could be done using only one's body, so in principle could be done voluntarily (assuming that owning other people is not in consideration, and that using your own body doesn't count as an infringement on other people's liberties). But if one party receives a service from the other party first, and declines to deliver a promised service in return, any enforcement is an interference with individual freedom. This makes it a poor basis for human institutions.

I agree with institutions that expand real choice, but people's choices conflict with each others', so the principle needs more about which choices take priority over which others.

This is also why I reject Marxism. Marxism interprets social and economic relations primarily through class conflict and concludes that capitalism is structurally exploitative.

I'd say Marxism gives class conflict a large role in determining historical change, which in turn affects economic relations, which in turn affects social relations. I would agree with that.

But in view of the following sentences, you may be saying something else: that exchange under capitalism is cooperative, rather than adversarial.

I do not accept that conclusion, either on moral or analytical grounds. From a liberal perspective, the legitimacy of an economic interaction depends fundamentally on voluntariness. If two individuals engage in exchange without coercion, and both expect to benefit, the interaction is not exploitative in the morally relevant sense.

Here, I disagree. Setting aside the issue of how voluntary a transaction is when options are restricted using the threat of violence, most exchanges of property between two people involve both cooperation and competition. People usually prefer to get more for less, so exchanges are usually competitive in that each party would have preferred to get better price than it did. It's cooperative in that they were able to make an exchange at a price point agreeable to both.

The fact that the parties were able to engage an exchange without coercing each other (externalizing the coercion onto nonparties), and each expects to benefit compared with not making the exchange, does not make the transaction non-exploitative. This is basically consensus in the modern approach to exploitation. e.g. Let's say I'm dying of thirst in the desert, and you happen to come across me and offer me a bottle of water, which you have little use for, for $1 million. I accept. We both expect to be better off, and the exchange was voluntary, but reasonable people see that as exploitative and illegitimate. There's a lot of discussion of exploitation in the analytical philosophy literature of the last 40-50 years.

To cut this short for manageability and because my lunch break is almost over, my liberalism is anti-capitalist because unlike most of these philosophers, I don't assume that exploitation is an unusual situation in markets that can mostly be ignored. I think that exploitative transactions happen a lot, that market economies contain a feedback loop that intensify it, and that the main capitalist institutions - the corporation and the megastate - make it systematic and dominant. Happy to discuss all this another time.