Scholastic Warehouse Sales. There are several sales in different warehouse locations in WA state. Scholastic sells a lot of great books - I highly recommend signing up for the sale on their site so you can get a coupon!
Once Sold Tales. Not only do they sell used books from their warehouse, but they sell them at $1.50 per pound! In case you're wondering - that's a GREAT deal. The best part is you can look up books on their site and then go with the list for them to find. Same price, but you get exactly what you're looking for. Apparently kids can get 5 free books a month there too. Not positive on that, but it wouldn't hurt to ask right?!
If you know of other locations (particularly in King County) where you can get great deals on books please share in the comments below.