I have seen this (listing multiple stacks of 99 of items that take time to accumulate via crafting, spiritbonding, or gathering) for a while and I've been swearing up and down to my FC that it is from item duping or botting.
In related news, some bonehead on Sargatanas (not going to name names) listed large stacks (55, 40-something, etc.) of Quickarm III materia for ~2K per item when it was going for 8K consistently. It immediately crashed the price to 2K and under as everyone else followed suit and undercut when they listed their individual pieces and single-digit stacks. I saw the same retainer doing the same thing on another item, though I don't remember which item it was. I really don't know what their motive is to dupe/bot so many items and deliberately crash the price unless they are just trying to move a high volume of goods as quickly as possible, even if it does mean making less gil and screwing up the economy for themselves and everyone else in the long run.