Some fish just ain't right in the head much like some humans aren't. When you buy fish pay attention to what they are doing to determine if they are normal. Ask a person at the store to feed and see if they'll eat. I got a maroon clown that had been trapped in an overflow(brother bought a tank we found it when taking it down) for like a year. After being freed all it wanted to do was swim like it was stuck under my rocks. I traded it in.
As far as the person at the lfs
: Never trust any one persons advice in this hobby. Don't even trust me.
Over time experience will give you all the knowledge you need.
Having a skeptical stance will pay off.