Can't speak to good used toys, as we've only purchased a few at consignment sales. These sales have been better for second hand children's books.
W/r/t furniture, Craiglist has been great. I bought several used Pottery Barn Kids pieces there (e.g., changing table/dressing station, glider, rocking chair, magazine rack, shelves, bed frames and desk w/ chair) all in good condition for a fraction of the price.