Might be too much trouble for your crafter friend, but they CAN check the MB and look at the history right after their items sell. It will show you the name of the person who bought it, so when you see it get re-posted later, it should be easier to figure out who this person is who's deliberately buying items from you and then selling them for less.
Also, if the disgruntled crafters keep badgering you after you explain yourself and ask them to stop, then they're the real criminals here