This is the same any where you go, in any field of retail. Woodshops in London, booze shop in Texas or electronic components in Japan.
Most retail outlets don't train thier staff properly. They just want to make a sale. As a general rule of thumb, don't trust advice in these outlets unless you know an individual's expertese to be reliable. This is a bit of trial and error mostly I'm afraid.
The thing with the dead fish, it might be anything. Did you see any disease symptoms? It could be that particular fish was stressed because of kids banging the glass, or got caught in filter. It could also be something on part of the shop's responsibility, or it could be a fish breeder's problem. If the 'dead fish' thing becomes regular, try watching any patterns. The owner/staff might just not know what to feed that particular kind of fish, or how much.
It could be so many possibilties. Other people in this forum would undoubtedly have more intimate experience with such aquarium outlet issues.