I have some more information to make the issue clearer as it's not as simple as "player(s) buy stuff then re-list and undercut" (though that would be stupid anyways as you'd never make any money...).

Basically there is this... "faction" let's call it, that own a bunch of retainers between them, and work together to not only undercut, but also target specific individuals and attempt to drive them out of the market by using the craft sig feature to tarnish that crafter's rep. Essentially some of my crafter friends are now being associated with this "faction" when in reality they hate them just as much as everyone else. Other players don't see that though, they see "items crafted by XYZ, therefore XYZ is part of the problem", and as you can imagine... it's extremely bad for business.

I'm not really sure what to suggest for them. Blacklisting everyone doesn't solve the problem, and there's not much sense in reporting a player who's just upset either (unless they get into extremes and harassment, then by all means). If there truly isn't a way to remove craft signatures, then SE should really look into implementing a way to make it optional. Maybe the use of a special item that acts like a "Terebinth" for items, erasing the signature instead of the dye colour.