The trustworthiness of commodity providers is a key issue to motivate customers to participate in a market. Several Grid Market frameworks are being developed and they are not an exception in the need of demonstrate their trustworthiness to clients. This paper proposes a simple reputation mechanism for auctions within Grid Markets, which is easy to embed in old Grid solutions that do not provide reputation yet. Effectiveness of this mechanism is proved through simulation in different market scenarios, by showing the direct correlation between reputation and revenue of Grid service providers.