I've check the Bonecrafting Guild once this week and once last week and neither NPC listed above had any Crab Shells for sale. Could it be something they only sell when Windurst is not in first place?
They sell items, sure. But items traded in to them. I went to the bonecrafting guild once to see very little on sale bar some cheap ingredients. This also dictates their price - the fewer of an item they have, the more it costs.
Crab Shells and Scorpion Shells do appear in the bonecrafting guild from time to time, but often sell very quickly due to the fact they are used in numerous recipes.
As I learned, best place for some materials is the trusty Auction House...
I can understand if the item is listed yet sells out fast due to high demand. But on the Odin server in the few samplings I have made of the Bonecrafting Guild, they have never been listed for sale. This makes me wonder why that is. Maybe I need to sell a few Crab Shells to the Guild and see if Crab Shells are made available. I'm not quite sure of the mechanics of the vendors so maybe this is how they work.
the absolute best place to get these in ffxi is in castle ostroja garden. The crabs here called "cutters", drop 1 or 2 slices of land crab meat every time and every couple drop a shell.