Secondary Spectrum Auction (SSA) has been proposed as an effective approach to design spectrum sharing mechanism for dynamic spectrum access. However, due to the location-constrained spectrum interference among users, it is a great challenge to provide truthful auction with maximized spectrum utilization. Most previous SSA designs either fail in addressing truthfulness or cause loss on spectrum utilization. In this paper, we focus on providing truthful SSA with maximized spectrum utilization. In order to minimize the computational overhead involved in addressing location-constrained interference, we leverage the truthfulness by introducing approximate truthfulness. Moreover, we define a general spectrum auction model using linear programming. Based on this model, we further propose ETEX, a sealed-bid auction mechanism with approximate truthfulness. Theoretical analysis confirms that ETEX is able to achieve truthfulness in expectation with polynomial complexity. Extensive experimental results show that ETEX outperforms most popular truthful spectrum auctions in terms of social welfare, spectrum utilization and user satisfaction.