The key is to ignore CO2 and focus on the much more potent ones, because you can sell credits for equivalent tons of CO2.

See: en.wikipedia.org/wiki/File:Greenhouse_Gas_by_Sector.png

Step 1: Collect Nitrous Oxide in giant tanks. Step 2: Store the Nitrous Oxide Step 3: Get $$$ from CO2 market Step 4: Use portion of money to buy 30 year government backed securities to cover maintenance of the tanks/land. Keep the rest as profit.

The key is to get your storage/transportation to a minimum. High compression, or piped to the ocean floor, maybe.