We are not talking about "security". We are talking about the client having to wait for every input to be validated.
Example: you try to sell an item, the client sends the request to the server, wait, wait wait, the server sends it's okay and you see the item sold
While the client is waiting it doesn't allow any other item sold.
How it would be way better:
you try to sell an item, the item is gone right away, client sends the request to the server, on error the client gets a message and the action is undone (this should rarely happen as the client obviously also checks for untradeable, inventory space, etc.)
And of course you can sell another item right away without waiting.