In order to make a Do-Not-Buy list, you have to first determine why it is not a DNB. Is it because the animals listed are hard to keep or impossible to keep? Moorish Idols are HARD to keep, but there are members here and I also know people who have kept them long term with great success.
What about Parrot fish, large Groupers, Sharks? Obviously very large fish, however, again, there are people here who have tanks big enough for these fish. A lot of large fish are not hard to keep. In fact, many of them are quite hardy. The problem is, many of them get big enough to swallow the tank they are currently in whole.
What about fish that are hardy to water conditions, however their feeding habits are very hard to replicate? Mandarins and Cleaner Wrasses come to mind. Mandarins are hardy to water quality, but their main problem is starvation. Cleaner wrasses feed primaraly on parasites, and need to do so. Even the few aquarists who get their cleaners to eat other foods usually do not succeed with these (in a simular fashion as Mandarins).
As for invertabrates... Most starfish should go on a DNB list becuase they usually do not get the proper diet that they need. All those "reef safe" chocolate chip stars seem to do well... but for some reason, none of the corals are doing well... What about the "harmless" green serpent star?
Carnation Corals, many Gorgonians, and other filter feeders are sold every day to unsuspecting aquarists, only to die of starvation or taken over by algae. However, search this forum and you will find healthy, GROWING animals.
IMO, DNB lists are bad. First, they never work. No one really follows them.
Second, if a fish is hard to house in an aquarium, and NO ONE (not even the super-dee-duper aquarists) get one, no one will find out how to properly take care of it. It takes the study out of the hobby. Lets say someone figures out how to get cleaner wrasses to live years, and BREED. Then the collecting of cleaner wrasses drops, leaving the balance on reefs alone. Then we could restock reefs with these animals where they are in small numbers.
Third, a much better idea would be "Do not buy UNLESS...:
you have a large tank...
your tank is full of copepods...
you understand the full care procedures of this animal..."
Ok, I think I'm done now...
BUT I do have a side story. There is a store that has had a cleaner wrasse live for over 5 or 6 years now. It is not for sale. It is back in their QT area. Any fish that has any external problem is put into the 90 gallon tank, and pulled out a few days later, and put into a different QT tank, where very LIGHT medication is added.