hi
i don't claim what i say to be the absolute truth as i'm still learning on this topic
but what i have to say is from what i understand, they should be the same parasite causing the same symptoms
stress ich is like what the ppl at the shop told you, the fish is stressed by transportation to your home and then a new environment (physical, chemical and social stress)
all these make the fish immune system weak so it gets ich
but if it continues to eat well, it should get used to the system and get well
however, if conditions in the tank are not good, the fish may not recover and the ich worsen till the fish dies and possibly cause an outbreak
PS:this is what i conclude from the observations of my own fish. there maybe some discrepencies between what i believe and what others think
hope this helps
(btw, my experience with my fowlr is even if water conditions are good, introduction of a sick fish can still cause an outbreak
for what reasons i do not know.
all i know is with my fowlr, i've had 3 outbreaks due to introduction of sick fish and each time, i test the water and everything is ok
but then again, remember that this is MY experience, does not necessarily reflect a general consensus)