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 the authors 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. They begin with a simple linear model for these complementarities that was shown to be computationally difficult in earlier work. They give several approximation algorithms for this model.