How to sequester carbon cheaply: buy a coal mine!

• Each ton of burned coal produces 2.5 tons of carbon dioxide. 
• A coal mine sells for about $1/ton of coal reserves.  
• It costs about $100/ton to take the resulting carbon dioxide out of the air and sequester it.  

So, rather than burning coal and paying $100/ton to sequester the resulting CO2, why not buy the coal mine and close it, for only $0.40/ton of CO2.  

Am I missing something?

HT:  Marginal Revolution