I have gone through the steps of a common fishkeeper: buy tank set up from Wally World, read the acompanying book, buy the coolest-looking/ biggest fish available and throw 'em in. You make some mistakes and find out the hard way how much improvision can be standed before your fish die/ tank goes to ****.
Anyway, when I was a newbee I did all things LFS people told me to do. Now I only go to the local shops because 1. they are more knowlegable, 2. Even though they aren't always the cheapest priced, they USUALLY have better quality fish and supplies, and 3. THEY CARE.
I also worked in my favorite LFS and even though it meant loosing commission, I would sometimes dissuade people from purchasing something that wasn't right for their tank/lifestyle. The fish no-no's are a given, but if someone came in to buy an iguana, for example, and didn't know anything about them, I would sell them a book first.
Bottom line, some LFS are wonderful, some not so great, but if you're gonna stay in the fish hobby you learn who's who. But let's face it, some people who buy the kind of things like pacus, oscars, and catfish and put them in a 30 gallon reguardless of a fair warning, probably won't be in the fish hobby for long. Some people just don't care.