There's also the off chance that this was a genetic matter, axolotls have been known to have random deaths of which the causes are defects in their genetics, such as organs collapsing etc.
However I feel the most likely reasoning behind it is that the axolotl was having immensely poor care in that pet shop, judging from the information we have on them in this thread, they're obviously very mis-informed about the care of axolotls. With such a short amount of time between the pet shop and her death, I think it's highly probable that there was nothing you could have done to help, Hopefully the shop gave you some sort of guarantee? The only reason I can think of for them telling you not to feed her for 3 days, is that they may have fed her that day, not that it makes a difference as long as you watch their weight. In pet shops they tend to keep all of the tanks (minus cold water - marine - tropical) on the same filtration system (with the same water) so it's possible - if this was the case, that the axolotl could have had a disease for some time.
You seem like an enthusiastic and caring owner, i'm sure an axolotl will be glad to have you looking after them. Your best bet is to have a look in the "for sale" section of this site, there's often axolotls for sale.
I'm sorry that you had such a bad experience on starting with Lotls, I'm glad however that it doesn't seem to have put you off.