You're correct on the marketability. You either sell it to be released later defeating the purpose, or by hopefully sequestering it to help with extraction of fossil fuel, which again...maybe not worth it. To actually remove massive amounts of CO2 and permanently take it out of the cycle is akin to burying money.

I see the main problem not as the cost, but the scalability. Our best efforts so far don't even amount to a fraction of a percent. There have been recent developments that could help some, so that would be a percentage of our annual emissions. A long way to go when the preferable solution is to remove emission amounts not only being emitted, but past years' amounts too.