i know maybe 15 people who have JB4 and none of them including myself have bought them new.
i actually am on my 3rd one and i would never buy it new.
with any used part they could have been tampered with, but thats what you have eyes for.
Cobb is the same thing, your fairly safe, except for if they left it married to the previous car (which ussually doesnt happen bc then the owner would have issues with warranties etc).
in both instances....your pretty safe with used