I would not trust that LFS... A fish that had been stressed from a return, then re-bagged and taken to your tank that wasn't cycled, had no strength to tolerate any ammonia. The idea behind using a damsel to cycle with, is because they are usually hardy enough to endure the new tank cycle, a stressed out damsel couldn't. The LFS people you are dealing with don't care about the fish. I would purchase what I needed for upkeep of my tank, but I wouldn't trust them for any advice at all.
As for the damsel dying, consider it a happy casualty, it most likely would have killed your future fish purchases. When a damsel is returned, 99% of the time it's because of it being so aggressive and already killing or bullying the other tank mates.
Beaslbob...shame on you, nobody needs to use a live fish to cycle with, ghost feeding will do just fine to get the cycle going.
Just my .02, if anyone wanted a molly, they wouldn't have gone with a saltwater tank. If a person doesn't want a fish, they shouldn't purchase it, and put it in their tank for any reason.