I'll admit that I'm biased about the Book Trader because I work here, but I'm also a regular customer of this and many other used book shops in Philly. Each shop definitely has it's own style, and perhaps it's own philosophy. At the Book Trader, the philosophy is to have more books than there is shelf space for. I don't think it's intentional, but it sort of just happens. The result is a treasure hunting experience for the customer, which only sometimes yields successful results. A book loving person will probably not mind this. However, it can be a daunting task if you are looking for something specific, or if you are claustrophobic (there are towering stacks of books everywhere, some as tall as a person).
Pros: Tons of books. Usually a nice atmosphere.
Cons: I could take or leave the staff. You might not find what you're looking for.