I actually think lfs's shouldn't sell goldfish. OK, maybe thats a bit of an overstatement, but think a bit. Most people who buy goldfish are first time fish owners. They may have had one as a kid, whatever. But they're probably very inexperienced. Tank? Since they'll be a beginner they'll want a cheap tank. And small because they don't want it taking up too much room. What do they buy? A bowl. What do you keep in a bowl? Goldfish. What goldfish will they buy? The cheapest and probably the most 'traditional' looking, ie. a common.
Almost all shops sell commons, most sell fancies but not all. Commons should only belong in ponds IMO. Fancies still get to 6 inches plus, usually around 8. They're also highly messy. So, my point is...
Out of all the people who buy goldies, how many actually have the proper facilities for one?
My answer is very few. The main things that annoy me are 1) The fact that most people think/say (lfs workers, esp. chainstore) that goldfish grow to size of tank (don't argue, I can't be bothered), but they still tell you how big they get (at pets at home at least). Umm, if thats true, whats the point? 2) If you buy a similar sized tropical fish, such as a Malawi, they'll ask about tank size, tank mates etc. If you buy a goldfish, they might, but they'll not give two hoots.
*The above was a slight generallisation, but note it doesn't happen everywhere. Mainly chainstores.