Carbon credits are generally sold by emission reduction projects via bilateral negotiations behind closed doors.
Carbon credits can be bought on the ‘spot’ market, when the credits have been issued and are sitting in the project owner’s account on a carbon credit registry.
Carbon credits can also be bought on forward agreements, when the credits have not yet been issued. There is a degree of risk here, as the project may not be able to issue the credits. The buyers typically expect some sort of discount for the risk they take or will have clauses allowing them to exit from the deal.
There is considerable investments in carbon credit exchanges around the world, but limited traction to date. Liquidity remains low and the ‘spread’ between buyers and sellers is excessive.