In fact, it is, so long as it's under enough pressure, and in the right place. In Montezuma County, Colorado, sits the McElmo dome, an ancient underground CO2 well. They pump it out, down a 500 mile pipeline, to Denver City, Texas, where it gooses oil wells into pumping more crude out. Other than making more oil and making it cheaper, not really much in terms of greenhouse gas contributions- the CO2 starts underground and ends up underground.
Kinder Morgan won't just let you back up your truck and buy some (it's already spoken for), and even if they would, they'd expect you to pay a pretty penny for what we widely consider to be waste gas.
I think MIT is doing some good work. Just wanted everyone to be mindful of the massive scale under which CO2 is already getting bought and sold.