I am buying clay blocks (not balls) at $9 / stack. There is a chest shop in the Block Importers tent in Terraferma to accommodate this.
THIS CHEST DOES NOT SELL CLAY!!!
I need tons for a major project I'm planning. If you need money & excavation skill, go out in the ocean with a full suit of gold armor (provides unlimited underwater breathing), 3 diamond shovels, and some food. 4 clay balls in a 2x2 pattern make a clay block. You can fill up your inventory and make $225 or more every time, courtesy of me. There are no aggressive mobs under the ocean.
I am usually on at least 2x a day to empty the box and make sure I have sufficient funds. If the chest is full, I will buy your clay the next time I am on.