Mobile ad hoc networks (MANETs) present similarities with Peer to Peer (P2P) systems in terms of decentralization, equality and autonomy. This results in the rise of new collaborative and spontaneous P2P applications over MANETs such as P2P auctions. The deployment of auctions over MANETS should provide new opportunities to the important marketplace created by mobile communities and may be a game changer in local markets when the mobile infrastructures are very pervasive. This business scenario implies multiple advantages such as ubiquity, availability, affordability, opportunity and spontaneity. Nevertheless, it faces various challenges such as fairness and security. In this paper we present a fully distributed architecture and a process designed to support P2P auctions over MANETs. The major goal of this paper is three-fold: i) to propose an estimation model of a fair round duration called Tfair (the bids collection duration) to fulfill fairness, ii) to propose a new scheme for access control and no replay that takes into consideration the unique constraints imposed by the crossing of MANETs, P2P and auctions, and iii) to validate the proposals by simulations. We prove the effectiveness of the proposed Tfair estimation model. We also show that the overhead of the proposed security solution is acceptable for the studied scenarios.