Essentially what happens is that when you join the cooperative, you make an investment into the cooperative. Basically you do that with par-value shares. You then get your profits built up over the year, which are, I would note, taxed as earned income. Unlike a corporation, when I get my profits from the worker co-op I'm actually paying earned income tax on it. It may be retained as shares in the cooperative, and then when I leave the cooperative the cooperative buys back all of my shares that I've built up through my initial investment and my profit-sharing.
As I said earlier, there's no capital gains because they're par-value shares when they're issued to you. You purchase them after tax, and you receive them without any tax benefit.