In the shop. Right now we don't sell used music online except occasionally on eBay.
A few music links:
All-Music Guide (actually it is now called the All-Media Guide with All-Music, All-Classical, All-Movie and All-Game divisions). A great source for biographies and discographies. Ratings and reviews as well but the quality is uneven but no worse than unimaginative.
BMG Music Service and Columbia House I always thought the record clubs were worthless. But once you can search and browse their online inventory and learn that everything is always on sale (50% - 75% not counting clearance sales) the picture changes. They each have their relative strengths and weaknesses. If you are a friend of mine & decide to join either let me send you an invitational email. I'll get free CDs and you should never let a conglomerate keep an extra penny.
GEMM lets you search the inventory of hundreds of music sellers. Probably the best way to find out of print music at the best price. They've recently added books and videos but those listings are slender.