>How would a libertarian society deal with unions
Unions exist for two reasons:
1) Because workers are legitimately fed up with management and are resorting to collective bargaining due to these discrepancies since it would cost the business more to train an entire new staff than to meet the union's demands. These are considered "legitimate" unions because they typically form, set a few goals, and move on. The business owner is not required to follow up with them, he can tell them to get fucked, but he has an incentive to work with them. This typically happens in businesses where the owner is too old to actually teach new staff or in fields that are more expensive to train someone off the street to do. This is where the meme of "that employee could fuck my wife and I couldn't do anything about it" comes from.
2) Because the government said you as a business have to deal with one if your workers form a union. These ones suck, are never opt-in, and are as corrupt as any government organization.
When you take power away from government to manage and regulate unions, the shitty teachers unions tend to disappear in short order while the unions actually accomplishing goals tend to stick around.