Generally there are other factors like how many titles in the series (as opposed to how many units each iteration has sold) and character popularity, but while sales doesn't directly indicate which characters will be included, it does offer a sort of rough display of the audience size, which does effect inclusion to an extent, so yes, listing them by sales isn't entirely pointless... though it's hardly the be an and end all.wait can't you just go buy how many copies they sell but I say we make one for fun i'll start
Mario is 1 and Pokemon is 2 (take that Zelda fans)
But yeah... the list (at least the beginning of the list) would probably look close to this:
1. Mario
2. Pokemon
3. Wii
4. Zelda
5. DK
6. Wii Fit
7. Kirby
8. Nintendogs
9. Metroid
10. Animal Crossing
(not counting Smash itself)