In West Chester there's two places I can think of off the top of my head and there's surely a few more. Fennario's is a coffee shop off of Church Street (right after it becomes one way) where there's some decent shows every once in a while. Check it out if you're ever there at night. I know they have a website, but I can't remember what it is and it took me a while to find it last time. They also make really good chai.
The Mad Platter is right near...Chestnut? on Gay Street. The address, according to the internet, is 111 West Gay Street. I haven't been there recently but I remember them having a half decent selection.
My personal favorite is Chester County Book And Music right off Paoli Pike. If you take Rt. 202 South it'll be on your right, directly before the Rt. 3 exit. There's a large shopping center on the right (you can see it from 202) and it's in the center of that. Take a right off the exit ramp and another right directly into the shopping center. It's a large indepenent book and music store (it used to be just books but they bought the Rainbow Records next door and opened their own music store). They have $7 used cds (with the occasional treasure inside. It takes a while to sort through it all but it's worth it) and a really good selection of music. They're also willing to order stuff for you, last I checked. Plus it's not actually in West Chester, so there's free and easy parking.
Re: Philly -- Spaceboy Records (part of Zipperhead) has some good vinyl stuff and sometimes free posters; they also have a local music section. It's on South Street, along with a couple others that I can't remember the name of. The Tower Records there has a ticketmaster inside, but otherwise it sucks.
Mmm...not much else I can think of other than good luck and make sure you stop by Jim's Cheesesteak on South Street during your near Philly cd shopping spree.