I seem to recall that Robert Zubrin's book "The Case for Mars" suggested selling Martian deuterium to Earth. That idea is tied with "mine the Moon for helium 3" as the least plausible business idea that otherwise-smart people can suggest with a straight face.
Deuterium, the heavy isotope of hydrogen currently valued at $10,000 per kilogram, is five times more common on Mars than it is on Earth. Deuterium has its applications today, but it is also the basic fuel for fusion reactors, and in the future when such systems come into play as a major foundation of Earth’s energy economy, the market for deuterium will expand greatly.
Martian colonists will be able to use rocket hoppers using locally produced propellants to lift such resources from the Martian surface to Mars’ moon Phobos, where an electromagnetic catapult can be enplaced capable of firing the cargo off to Earth for export. ...
EDIT: he also shared this idea in articles.
https://space.nss.org/the-promise-of-mars-by-robert-zubrin/
Deuterium, the heavy isotope of hydrogen currently valued at $10,000 per kilogram, is five times more common on Mars than it is on Earth. Deuterium has its applications today, but it is also the basic fuel for fusion reactors, and in the future when such systems come into play as a major foundation of Earth’s energy economy, the market for deuterium will expand greatly.
Martian colonists will be able to use rocket hoppers using locally produced propellants to lift such resources from the Martian surface to Mars’ moon Phobos, where an electromagnetic catapult can be enplaced capable of firing the cargo off to Earth for export. ...