Make a way to sell your extra produce and minerals for cash. This will help early games out but not really have much of an effect on end game without drastically altering the games mechanics. New players also would not feel out of money all the time being unable to do anything.
A better Way to make money Early game.
You dont need to sell your materials to gain access to early money.
You can Join an abondoned alliance name it what you wish and deposit money into it and reset and repeat.
Your idea about making money from materials is a very good one though.