Whereas the lfs if very much responsible, adn staff should make a point of knowing about the fish they are selling and should be asking what fish people have in their tanks, it is no only down to them.
People buying should make a point of telling the staff what they have, fish and tank-wise.
If the buyer is looking at a fish that they have no knowledge of prior to purchase, they should ask the relevant questions, such as "is it aggressive?". If the staff do not supply a convincing answer, ask if they have a book to consult, or better yet, do not buy the fish until you have had a chance to read up on it.
It is very easy to keep blaming the fish stores, but as much of the blame lies with people doing the buying. Sorry if that does not sit well, but it is true.