It annoys me how much benefit of the doubt people give others when it comes to bots, because if I suspect someone, I'll investigate thoroughly before I start making conclusions.
This player in question is not just gathering and undercutting constantly, he's always exhibiting the usual janky repetitive movement patterns that are typical of a bot. I didn't spend excessive amounts of time watching him in person (I do, however, monitor suspect players with player search), but every almost single time I've been in Limsa Lominsa Lower Decks for the past week or so over the course of my somewhat excessively long play sessions he'd be sitting by the retainer bell and if he wasn't, he'd show up within about three minutes if I waited there.
While you can find his price floor and sell lower then him for a time, he'll eventually notice and drop it even lower, so you really can't win. There's also the fact that he's only one of multiple bots doing the exact same thing, so even if you can steal his sales away it's not uncommon for one of the others to start picking up his slack because they seem to operate in different time zones.
Craft bots are common, but people who use undercut bots are not. This player is only the second one I've noticed on my server; the other having been a longtime nuisance since the late HW days and whose name has become familiar to pretty much all of the legitimate crafters on the server due to how obnoxious they are. They play it safe by either using alt accounts to farm their mats or just buying it all straight off the MB and are holed up in their house crafting away about 99% of the time.
Some of them also do their selling on alt accounts, possibly to reduce the risk of their entire operation getting shut down if they do ever get caught. The latter one I mentioned has a trio of low leveled alts in their FC stationed in each of the three ARR cities that log on/off in a constant cycle whenever they're actively undercutting people.