But if Uber isn't yet public, and won't allow a market in its shares, who would one sell the shares TO to cover the tax?
I think the sibling post answered the question -- RSU's don't vest until the company goes public. So, instead of getting illiquid comp, you just get none until Uber is public.
Typically this is only for short periods for higher-up people but it can affect "normal" engineers too (say you do a tech due diligence for an acquisition etc....)