Lesson number 1: never listen to a salesperson in a lfs.
It is obviously up to you but I'm sure other members will agree that it's just really too small for the goldfish. Have a look through some of the other posts about keeping goldfish in small aquariums and you will see.
It is very sad I agree :( There is a sort of rule that a lot of fishkeepers use: 1" per gallon rule. It's not a failsafe method and has a few holes in it IMHO but basically it says that you should have 1" of fish (adult size) per gallon. There are certain times where tis can be broken. For example with small tetras or minnows because they are such small waste producers and also because they have a long slim body meaning that they don't add up to an inch squared kinda thing. On the other hand there are some fish that break the rules in the opposite direction such as goldfish or plecs because of the hiugh levels of waste, or particularly deep bodied fish such as angels. Like I said it's not failsafe but it gives you a rough idea of the types/numbers of fish that can go in certain tanks.