In cap-and-trade system, countries can buy and sell
vehicles.
businesses.
fertilizer.
pollution credits.

In cap-and-trade system, countries can buy and sell pollution credits.

Carbon credits is a permit which allows either a company or a country to emit greenhouses but limits the amount that can be emitted. It aims to reduce pollution and greenhouses gases emitted. The credits are valued against one tonne of carbon dioxide and a certain number is allotted to each country.  

Trading carbon credits between countries or companies promote balance in worldwide emission. The proposal for pollution credits that set emission budget for countries was made in 1997 and meant to be in operation in 2005. Participating countries signed to endorse the deal.