Let me first start out by saying quality is definitely objective. There is the general populace that agrees with what quality is but there are other outlying people that would disagree with what the general populace thinks.
Sales do equal quality but it certainly isn't the only factor.
I would say the two biggest factors of sales are brand name and quality depending on where the game or franchise is within it's life. When a game first starts out, quality is it's biggest sale factor but when it becomes an established franchise brand name becomes the key factor in sales. It's also important to note that brand name and quality can feed each other with each game to make a super franchise such as COD.
Here is what I would say are the biggest factors of sales for new IP's and established franchises (I won't be adding genres because different genres have different caps on sales expectations). I'm probably missing a factor or two here.
New IP's: quality > advertising > word of mouth > what console it is on in any given region (different regions generally have slightly altered taste) > other similar games in that genre on the same console > what studio developed it (which is the only form of brand name a new IP really gets).
Established Franchises: brand name > quality > advertising > word of mouth/it's whats "in" (think all my friends have it) > what console it is on in any given region (different regions generally have slightly altered taste) > other similar games in that genre on the same console
