It depends on how you define quality. It also depends a lot of the machine it's on, and why it' selling.
I think you could say that any title selling over 10 million is a high quality title in the eyes of the people buying it. You have so many people choosing that game over any other game that they have to have a good reason to really want that game.
So I'd say 10 million is the universal number, and also the Wii/DS number.
If it's an exclusive to the Ps3, X360 or the PsP, the number is obviously far lower. If it's a Ps360 multiplatform, the number is probably around 8 million or so, and if it's an all platforms multiplat, then it becomes really tough.
I think that it's completely fair to say that in a sense, sales = quality in the buyer's eyes. If a game sells 10 million, it has to have a very wide appeal, and thus appeals to mostly everyone. There's obviously also some people who dislike it, but they're normally far and few between. There are obviously certain exceptions to this - Wii Play might be one.
Now, that's not to say that a game that sells little can't be quality, and that's not what Galaki is saying. Games such as Okami can be brilliant to the people who play them, but they could never have universal appraise, because they're so unusual.
A game is a quality title if people like it. Games that many people have bought are thus more likely to be "quality titles", because many people have thought they would like it and bought it.
