This level 72 RDM was in Qufim explaining to low levels that crabs can be killed by anyone since he needed to get lots of rock salt, and couldn't get off the ah.
He further explained that there is NO npc in the game that sells it and wiki doesn't say anything about it being sold at NPC's.
I'm here to propose that 5000+ people send him 1 rock salt each >.>
http://wiki.ffxiclopedia.org/wiki/Rock_Salt <--- Might help if you read this Madgamer.
Btw, since you mentioned ingame its your $15 dollars a month, you might want to invest in WoW so there's a question mark on the heads of NPC's that sell what you can't find/afford on ah.