As there is an obvious realtion beetween the number of gemstones given to the npc and the nuber of armbraces isn't there, by any chance, the possibility to know how many armbraces where traded? then Anet/Ncsoft could have a ratio of true ones and dupped ones.
something like:
15 gemsets = 1 armbrace
xxxxxxxx armbraces traded @ npc
yyyyy tormented weapons out there
now, xxxxxxxx-yyyyyy = nº of legit armbraces tradeable
finally, zzzzzzzz- nº of legit armbraces tradeable= nº of rogue Armbraces
(not counting with the rogue ones that were traded before by tormented weapons...)
its all in the DB
honnestly, i think its not worth the trouble.
consumables and ub do the trick. as Gaile said, its pure economics - offer & demand.