A consumer is someone who buys or consumes, or uses up, something. An economy could not keep functioning without consumers.
Long before smorgasbords, shopping malls, or eBay, people craved pretty and tasty things. So those who spoke Latin invented the word consumere, which meant to use up, eat, or waste. Being a consumer doesn't necessarily mean you are squandering. We're all consumers when we go through the checkout line at the grocery store, for instance.