The money in play between the two is first 20 & 20, then a 30 comes into play as the box with 40 in it is sold for the former price, this brings the total money in play to 70, divided between the two would bring in a “profit” of 15 on top of the initial 20.

So not only the math for the profits is wrong, but it only works so as long as the person buying the box keeps contributing money and the person selling the box is willing to combine the total money exchanged and divide it equally between the two.

You’d literally have an easier time saying “Hey dude, lets pool our wealth together to increase our collective buying power” instead of trying to make money off of malarkey