An important cloud federation enabler is the development of suitable business models, which has so far received relatively little attention from investigators. Few efforts have been dedicated to investigation of business models for enabling cloud federations, especially with regard to distribution issues, in which centralized solutions are usually preferred. Hence, in our work we focus on the use of fully decentralized mechanisms supporting federation of private clouds based on barter mechanisms. We analyze the adaptation of fully decentralized incentive mechanisms previously used in the context of P2P desktop grids, and show that they are not suitable for federated cloud systems because fairness cannot always be guaranteed. We show initial results concerning the use of a mechanism intended to guarantee a higher level of fairness and thereby to promote voluntary participation in a decentralized federation without any central or trusted enforcing authority.