The fact is, these people are not selling the items less than cost. At least, not THEIR cost. They farm tomes rather than buy philo mats.

As well, if you buy Philo mats in bulk from a person(10+ at a time) you can usually get them to drop the price even more than Market board(which anyone shouting to sell mats is already selling lower than MB in the first place). Many people would take say 32-35K NOW, then wait to sell the other 2 mats to make a full 40K, just because it's convenient.