My local supermarket has had zero days loss in trading in the last quarter. What does it do? It buys things for a cheap price and sells them at a higher price

My local supermarket has had zero days loss in trading in the last quarter. What does it do? It buys things for a cheap price and sells them at a higher price. Traders do exactly this, it is the service they provide. It is quite unlikely that all the traders in a big bank are going to lose money on the same day.