They money I used to buy a candy bar is post-tax dollars dollars, but I still pay sales tax on it. They money I use to pay for a taxi ride are post-tax dollars, but the taxi driver still pays income tax on it. When I sell that stock I bought with post-tax dollars, I still pay capital gain taxes on the profits.

Post-tax dollars aren't magical things that mean you never pay taxes again. They're just an accounting tool that makes dealing with some tax-exempt activities easier.