Newbie, I would resist the temptation to just categorically blame fish deaths and disease on the LFS. While we all know there are lots of cases where this is accurate, it's much too easy for us (especially beginners and friends who sound credible) to attribute dead fish to the improper care that was given by somebody else.... which means we stop looking for ways to improve the care we give them. It's very difficult to figure out everything all at once and, sadly, there's no way for anyone to really get into this without experiencing some losses. In most cases, however, even a fairly sick fish we bring home can be treated and has a good chance for recovery if the problem hasn't progressed already beyond hope. This can be very tricky, of course, and depends a lot on us knowing what to do and what not to do... who to listen to for advice and who we shouldn't.
I've had more than my share of inexplicable deaths and I can surely tell you, it's impossible to forget them. As I keep working to learn more about this myself, I still think back to the fish I've lost in the past and wonder what was wrong with them... what else might I have done to save them?