Many mars naysayers claim the colonists on mars will all die when they run out of some critical supply item. First, staying alive is an individual responsibility. Second, the general store and basic economics insures this does not happen.
The general store performs a number of beneficial functions. It buys in bulk and sells items at a markup. This provides a buyer for those that produce stuff. This provides a buffer of items for before they need replacement. It balances supply and demand. Competition establishes the best rates and a further supply buffer.
This is so obvious it shouldn't need a post, but naysayers never seem to acknowledge this... because they don't believe in free enterprise.