Unless you don't plan on shopping there, just say that you use "liquid chlorine" which is what bleach is (typically 5-6%) and also what they probably sell too (typically 10-12%). (You can check the ingredients to prove this to yourself if you have doubts.) Otherwise, it's not worth getting into the discussion - I doubt someone is going to say "hey bleach is the same stuff and a lot cheaper so don't buy my checmicals just go to the grocery store!"

Also, to be fair, I don't know what kind of spiel these sales people get from the chemical companies so it may not be entirely their fault. They may just be repeating what they have been told.

The important thing is for you to know what's going on and only buy what you need.

Good luck!

Peter