"from a private third party" where? A (non-foolish) socialist would advocate for rules against renting people, just like we're not allowed to buy people right now.
That would mean there would be no private third parties that are renting out factories of rented workers.
If what you're saying is "from a private third party outside the socialist space", then that's a problem for all kinds of socialist spaces. We can't control productive forces outside of the space we have domain over.
Over 85% of Mozilla's income comes from their Google search deal. Google is keeping Mozilla alive to prevent antitrust issues. If Mozilla rocks the boat too much, Google will fund a more obedient alternative.