Thanks for responding.

Not necessarily. You are assuming that if Bob doesn't employ Alice, Alice stays unemployed and sits there twiddling her thumbs. In the context of the whole economy that is not true. Alice is likely to go out and get hired by Eve, leading to her producing similar economic surplus. In that case, Carl does not need to account for Alice and estimates his contribution correctly.

The concept of opportunity cost already takes into account Alice's next best opportunity. The $15 figure I gave takes into account the other options she'd have if Bob didn't hire her.

Now, you may believe that for most people, the wage for a job is very close to the opportunity cost, i.e., there is little surplus to workers (this would be true if a lot of workers were competing for a job). In that case, my consideration doesn't apply, i.e., we don't have to worry much about displacing a worker because the worker wasn't getting a huge surplus from doing the job. The question of whether a particular job fits this scenario is empirical.

It's simple on a basic level. Carl and Bob together can produce some value. Whether Carl volunteers or is getting paid affects only how the economic surplus is divided between Carl and Bob. From the society's point of view it doesn't matter, the same economic surplus is produced.

That is correct, but the choice here isn't between Carl volunteering and getting paid, it's between Carl volunteering and not doing the job. See also my next paragraph.

And if you are arguing that Carl will crowd out Alice, well, so will Alice crowd out Deborah, another person who would like to work for Bob but cannot because the job is taken by Alice.

Yes, of course. But the market takes care of that by allowing people to bid on price. But when a person volunteers for free, they are in essence subsidizing themselves as workers, "picking winners" as it were, and therefore interfering with the market mechanism.

A lot of things may happen. Are you arguing that this case happens often enough in real life so that people should prefer not to volunteer?

In my comment, I noted that due to lower efficiency, the opportunity cost to volunteers for a given amount of production may be worse, even if they have lower opportunity cost per hour.

Volunteering does not second-guess the market mechanism. It's point, often, is precisely to compensate for shortcomings of the market allocation judging these shortcomings from the point of view of the one who is volunteering.

That's what I meant by second-guessing the market mechanism.

Note that you gave a fully general argument against any allocation of resources other than by the market. Are you sure you want to go there? :-D

Yes, I think there is a strong prior against interfering with the market mechanism. The prior can be overcome in many cases, but the first step to making a strong case for overcoming the prior is to first understand how the mechanism is operating. If you don't account for the welfare of particular members of the population when you do your analysis, you're likely to be led astray.