Why are there no private Minecraft or GTA servers where players do real money trading against game items is something that is legally forbidden?

Hello Why are there no private Minecraft or GTA servers where players do real money trading against ingame items is something prohibited by law because you have no business?

Because at the age of 12 you are not fully capable of doing business.

Say GTA 18, were on the private server, would the players have to sell each other items for real money?

There's still no crucial supervisory authority. It is not possible to guarantee that everyone is really 18. I can't answer the second question, but you should assume that something like this will never officially exist, as this would override the monetization system of, for example, Rockstar.

Hey yes I mean not only so private servers even if Rockstar would offer this well with age proof is true.