I have bought scraps, old blocks, an unfinished top and cut pieces. If you are NOT buying from a shop or regular vendor be sure to read the descriptions and squint at the pictures. Some dealers don't know as much about fabric as they might....especially about fabric content. Ask questions before bidding.
The charm packs I purchased from several sellers have been GREAT--perfectly cut, smooth, and assortments I would never been able to assemble myself.