All of Grasshoppers games sale like ****
Killer 7 Sold Like ****
No More Heroes sold like ****
No More Heroes 2: Desperate Struggle is getting made
Another non-wii example
Guilty Gear
Guilty Gear XX
Guilty Gear XX Slash
Guilty Gear XX Accent Core
All sold extremely poorly compared to King of Fighters and Street Fighter, not only do they continue to get made but they continued to be ported to other platforms, and have updates and was even licensed by Sony in recent days.
Now, regardless of that, the game still shouldn't care unless he's playing the games for another reason. Saying that great games don't sell on the Wii is not an argument for not buying it, as well into the 2010 we still see great games in the line up. If I play Katamari Damtichi I'm not going to stop and think, hey, this game didn't sell a Million, why am I playing this? This sucks. No, if I do that or have the opposite reaction to a game that is not a MMO (which relies on the community). Sales are really, really pointless, because they don't make the game any more or less fun. Because of a bad marketing campaign, Boy and His Blob is going to sell horribly, never the less, I find the game very fun. What should matter to the entertained is being entertained. Let the entertainer and producer worry about the sales.
Protip, sales are sales, money is money. A lot of games that did poor in the US sold well elsewhere and got a sequel made.