Oh, I just got it! $$$
I now finally understand how Gatsby made his money from the line Tom delivers as they run out of the Plaza.
There was an exception to Prohibition - you could sell alcohol "legally" as long as it was in a drugstore and the person presented a prescription. Very much like medical marijuana, with fake prescriptions and pure alcohol, except that this was really a legal operation.
However, I'm sure that he got the alcohol illegally and there were tons of questionable deals involved with that.
But what was Tom talking about with Gatsby's new deal with Wolfsheim that no one wanted to talk about? I wonder what that was.
I want the doctor to take your picture so I can look at you from inside as well.