Most of the large market playing crafter's probably know who you are speaking of. The player(s) have at least 18+ retainers associated with their crafts(all of which are selling near capacity 24/7). They quite possibly could have taken the time to level all their character slots for the account to at least 15 to have that many slots, who knows. We probably wont ever know.

All you can do is report them, if they are doing something wrong SE will hopefully sort it out in time. Until then they get to spam all of the Militia / Imperial Operative / Crafting and Gathering class sets.