Extensive spectrum markets have the potential to enable more efficient use of this limited resource. Such markets must account for particular properties of the underlying wireless medium. In this paper we focus on one such aspect: the role of interference created among different agents who may purchase the right to use the same spectrum at nearby locations. Such interference can result in complementarities among the spectrum goods being traded, which complicates the design of an efficient market. We begin with a simple linear model for these complementarities that was shown to be computationally difficult in earlier work. We give several approximation algorithms for this model. We then consider several alternative models in which the spectrum goods are defined in different ways and explore the impact of these choices on the complexity of the resulting market.