Too bad for some items, such as the ones that sell once per day, the market changes enough that what is YOUR break even price (with materials bought 5 days ago) isn't another player's break even point (materials bought today). I had what looks like a player like you buying up my stuff at what they thought was a loss for me. While I was getting a consistent stream of 500k-1M gil from a set of artisan or forager's just from quick sales to these guys. They were probably all smug about it. I'd just make another set of gear in a few minutes, put it back up for sale again.
They eventually stopped buying up my stuff and sold at my price instead (and then of course undercut by 1gil each, since even you 100k undercutters can only go so far).
IMO, that system only works if your competition doesn't actually create a new item again.