There are a couple of places in old Ellicott City that sell used LEGO. There used to be a vendor at the fleamarket in Dundalk too. The flea market was less expensive, but the parts were all common. I've had better luck being patient and buying lots off of eBay, but if there's something specific I am looking for then Bricklink.

Most people are aware that LEGO hold their value, it's difficult to find them cheap especially if they are complete sets. The only true scores I've gotten is when coworkers who know I am into LEGO have just gifted me their kid's old sets.