You can buy and sell almost anything under capitalism. You can buy or sell property. You can buy or sell pieces of a company (stocks). You can buy or sell human labor. And the way that businesses make money is by essentially arbitraging some market, for example by buying something in one place and selling it somewhere where it is more expensive, or by buying labor and selling the products of that labor at a profit.

But there are certain things you already cannot buy and sell in our current system, and this is key to seeing the way forward. You can't buy and sell people, for instance. That would be slavery. The purest form of capitalism would include slavery also. You can buy labor, but no more than 40 hours per week without paying overtime.

The socialist point of view on this is that it isn't just buying the life of a person that is morally wrong; buying their lives piecemeal 8 hours at a time and then arbitraging the value of their labor is wrong as well. So under socialism when people work, they work always and only for themselves or as part of a cooperative that they are an equal owner of. So they are taking part in a voluntary activity, not selling themselves. Ultimately that's the only way to fix our economic system; allowing the arbitrage of labor leads to fundamental instabilities and inequalities that cannot be resolved. Otherwise the pressures within society will continue to grow until the society tears itself apart or relieves the pressure by going to war.