The credits for store purchases would be so cheap as to be pretty meaningless. At the pump or in the home heating and electric bill it would be a different story.

Of course in mandatory credit schemes, buying them is not an option, you can't burn the fuel at all without credits. Usually these get applied to the big vendors who then pass on the cost.

It's 113 gallons to a tonne of CO2, or 8.8kg per gallon. At the Euro price that's 10 cents per gallon ($11/tonne) and with a 20% markup that's closer to $2 on a 15 gallon ($50) fillup, which I could see people doing. Of course with so much demand the price would go up, as it should. And there would need to be more regulations to stop scamming by sellers.