in 12 years of using Photoshop I have never purchased a brush set, I imagine that when you reach the level of knowing to look for brushes, you will also have reached the level of knowing how to build them for yourself.
Lots of site sell them though, someone must be buying them. Ive seen them for sale on DeviantArt - its cheap or free to list ther - also on photoshop forums
It is free to list on DeviantArt but I haven't found a way of charging for them. Maybe I should just be happy knowing that I can make my own and don't have to buy from anyone, would be nice to make some money from it tho