From what I read so far, I'd like to caution you with some of the faulty debate arguments. Popularity =/= Quality. For example, here in QC, I usually get very high grade tea from a shop called Camelia Sinensis. The taste is awesome, but it's not cheap. There is a second shop, David's tea, which is more popular because it's cheaper, but the tea is not fresh and they put a bunch of unhealthy/foreign elements that compromise the quality of the product (E.g. candy canes). The quality is horrible, but it's accessible. It's like how casual games sell well. Objectively-speaking, they are often short and shallow. It will sell much but won't have many diehard fans. I don't know many people who call Wii Sports their favourite game, and that sold VERY well in Japan (It wasn't included with the system). Many games that sell well are games people kinda enjoy but aren't crazy about.
In other words, sales could be used to infer the number of people who wished to buy the game (For themselves of for others). However, many elements remain unknown.
1) Did the person actually like it?
I bought Super Smash Bros Brawl and was so dissatisfied with it that I played four matches and put it back in my library to collect dust until I found out about Project M. I bought Brawl, but I hated it. Infering "liking" from sales is faulty.
2) The degree of liking.
I buy about one game per month. That makes about 12 per year. Among those are games I like but I'm not gung-ho about.
3) Unsold titles.
Piracy. I don't need to say more.
4) The average person is not a specialist.
It's like my tea argument. People like tea bags. Unless they try actual tea leaves, they will continue to love tea bags. When they try actual leaves, many won't want to go back to tea bags, because they experienced a better taste. An average casual gamer doesn't care much for "quality" and depth. Heck, the kind of game matters little. Games are meant to kill time for these people. Not that they are idiots. They just don't care.
5) You can like a game that is not objectively good.
I got a friend who is a NIS junkie. He loves the likes of Disgaea and Makai Kingdom. They are very basic, generic J-SRPGs. To get everything, you must grind, grind, and grind again. That is poor game design. Does it mean he's a fool? Not really, but he likes a game with objectively observable flaws.
6) Generational gaps.
New gens games, who I call 90's kids (Jokingly), are spoiled with new-gen controls. When they go back and try SNES games, it's the same as when we go back and try Collecovision. They're like "Dude, no dual analogue? No 3D? What gives, man?'. Chances are they never even played it. Currently, to pool of "gamers" has increased, so a sales "success" today is different from a sales success from back then.
7) Used games.
More poeple played the games than the actual sales. Used games don't get registered in stats. Hell, my friend loved Melee but didn't have a Gamecube (So he got Project M). So he played only at my place.
In sum, using sales to infer quality is overall a faulty argument. Even if there may be a possible correlation, one cannot infer causation from a correlation... we can merely say that they move together, thus are likely associated. Even that is uncertain because an association can be very indirect.