I work at a LFS...
And most of the cases when a customer wants a fish, i tell them what I recommend based on what I know, or i look it up for them. But if a customer has made their mind set on something, they usually do it regardless of my advice, and if i tell them i don't want to sell it to them, they get all preachy.
One common thing with customers in my experience is that, if there is a fish they want that will not thrive in their small tanks, they allways say they will upgrade the tank soon. Based on the amount of people who tries to return the fish when they've outgrown the tank i've divided the displays in two.
One half of the tanks displayed is for beginners with fish that are all compatible and will do well in the smallest tank we sell, next half is for more advanced i say, but still rather easy, but wont go well in all cases with eachother or requires bigger tank, like nicaragua cichlids and such.