I buy a lot of my stuff through Noble Knight Games, but not because they're cheap (they sometimes run on the steep side, in fact) — because the service is
stellar. They always ship very quickly, the books are always as advertised, and if they have an order error they'll bend over backwards to make things right.
As for ridiculously cheap books, Troll & Toad has pretty much become my first stop for those — I picked up all of the WotA core books in Like New condition for $5 a piece, the entire line of Agone books (plus four extra core books) in Mint Condition for less than $25, and copies of the Authority RPG in Near Mint condition for $5 a piece. Good times
