This thread isn't a Halo vs Killzone thread, this was just as an example. It's acceptable to talk about these games in context of the original question ("does quality = sales?") but if it turns into a Halo vs Killzone flamewar then it will be locked, and suitable moderator action will be taken against those responsible.
I've just discovered something shocking while browsing through the vgchartz database...quality does not equal sales, and I have definitive proof. Halo 4 and Killzone 2 are the best examples I can use to illustrate this phenomenon because they're both their respective consoles flagship exclusive FPS franchise.
Anyway, lets look at quality first. Here are the metascores of the games:
Killzone 2: 91
Halo 4: 87
Given that Killzone 2 is objectively better than Halo 4, any rational person would assume that Killzone 2 has sold more than Halo 4 ever will... But that will certainly not be the case. Here are the sales:
Killzone 2: 2.87m
Halo 4 (estimated LT sales based on a secret formula): 10.9m
That's right folks, despite Killzone 2 being of a higher quality, it has still sold less than Halo 4 will. Halo 4 will even outsell Killzone 2 FW only. What the hell?! Startling, to say the least. Can anyone explain this phenomenon?