Rasputin Music, located on Telegraph Avenue in Berkeley, is a great place to shop for used and new music.
They have two floors full of new and used cd's, and they have a large back room filled with DVD's. Their used music prices are fair, and I rated them as "good" because their new music is priced a bit steeply sometimes.
They have a helpful staff, and also have computers set up around the store to help you find or special order something if you like.
They also let you listen to something before you purchase it, which is rare nowadays.
Be sure to get the "Rasputin Card", which will enable you to get a $1-2 discount on their large catalog of used music.
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