The goal of Supermarket(SM) is to provide products to customers and a stable occupation for those working in the system, not to maximize profit. SM will only sell to subscribed members.
Whenever someone buys something, they pay only the cost of the product, and the purchase is recorded. At the end of each month, SM has to balance its finances: costs have to be equalized with income. We will charge our members an additional % over whatever they bought for this month, just as much as it's needed to balance the books. The more customers we have and the more volume we moved, the less % everyone has to pay.
This way, instead of customers paying a predetermined % margin for each product, they pay as little as it's needed to keep things going. They don't pay for anyone's profit.