The bottom line is that they're cheating and making it more difficult for actual players to make gil off the marketboard by tanking prices to a point of scarcely being worth the effort and exasperating the issue with automated undercuts that require one to be extremely if not excessively diligent in monitoring your prices to make sure anything actually sells.

Not to mention many of them are also involved with RMT and using the marketboard as a "sustainable" method of farming gil as in all likelyhood a lot of the gil they sell ends up coming right back to them from the people buying their crafts, plus they almost never get banned unlike the hacking ARR gil farmers.