It's probably not economic to ship unprocessed ore back to Earth, so there's quite a lot of tools and equipment that will have to be produced on-site or shipped from Earth.

Mars doesn't have a lot of free oxygen in the atmosphere, so even with local methane you're probably not going to be able to use chemical fuels for mining or industrial production. The obvious energy sources are solar and fission, but both have their drawbacks.

I think the key to human utilization of Martian resources will be (approximate) self-sufficiency. If humans on Mars can locally produce industrial equipment, food, and (nuclear) energy, it's likely that they can create something of sufficient economic value to trade with Earth.

If any of these three are lacking, this implies a continuous flow of resources from Earth.