Cloud Computing represents a new era where computing is offered as a service rather than as a physical product. The next level of flexibility will be achieved when Cloud Computing services can be automatically traded. This paper focuses on providing the foundation for simple and flexible Cloud resource trading. This is achieved by proposing vocabularies for the trading of Cloud resources and algorithms for a Cloud marketplace. A multi-attribute combinatorial marketplace is proposed as a solution for situations where Cloud resources need to be traded in combination (bundles). Vocabularies are introduced to serve as a foundation to build standards for Cloud resources trading.