Magus is an independent used bookstore about a block south of University Book Store and around the corner. Their wide selection includes a larger music section than any other bookstore, new or used, out of dozens I’ve tried, not just in Seattle but also in the Bay Area—with very affordable prices on their orchestral scores.
The staff of Magus is good enough to let you alone to shop, but very pleasant when you need to talk to them. Though they only buy until 3 p.m., and on limited days, they pay about three times what Half-Price does, and in fact more than any other bookstore I’ve shopped at in Seattle or the Bay Area. Highly recommended.