You can only use ISK when selling or buying in game items.
ISK is the in game currency not PLEX. You buy everything in game (not counting NES) with ISK
Attempting to buy something with PLEX can make it look like you are attempting RMT. So to prevent that, items and characters are sold or bought using ISK only
Also to get an isd to answer you need to tag the actual isd like in your example
@ISD_Drew or @ISD_Traindriver. @isd does nothing