Platinum Records is located on E Pike St, one block East of Broadway on Capitol Hill. They specialize in vinyl records, DJ equipment, and PA systems.
For vinyl records, they have of the largest selection in Seattle of Top 40, Hip Hop, and Dance. There really is no comparison, you might be able to find smaller shops that specialize in a more specific genre, but overall, I think Platinum takes the cake. Although you should be prepared to spend some time in there, as they don't have a computer inventory system, and stuff is usually grouped by labels. They also have a decent selection of underground CD's and DVD's that you would not find in a typical store.
I would personally stay away from their DJ equipment, as you can easily find the same gear for cheaper prices either online or at Guitar Center. However, I would definitely check out their PA system rentals for your next social/work events.