I don't think he's an alt but a very dedicated crafter. I've seen the same thing in my server where one person comes in and starts to sell tier 2 gears for half the price of the previously current market. I came to the conclusion that that person had just transferred over from another server and since people who are transferring to another server are restricted to only have 3 million gil + 1 million gil for each retainer, many people that have more would have to buy tier 2 mats in their previous server in order to not lose their gils.

Of course they would have to recoup their gils back by crafting tier 2 gears and selling them at how much it cost them to buy the mats from their previous server, hence the low prices or what. I would have done the same if I had to change servers.