Nope, you're wrong. Sales are only a way of gauging mass opinion, and mass opinion is no more truth than just one person's opinion.
Like, Twilight, the movies. Both of them have made 100's of millions of dollars, does that make it good? No. Does that make it bad? No. It means there are fans who like it and find it good. It means there are people who hate it and find it bad.
Avatar, biggest grossing movie in ever. Yet it has a lot of haters. Does that mean it is bad? No. It doesn't mean it is good however either. It just means a large group of people found it good.
To move on to something different. Rebecca Black's Friday has 150+ million views on YouTube. Does that mean it is good? No. Does the mass amount of people who hate it mean it is bad? No. Some people enjoy it, most people don't. But it does not change whether it is good or bad.
Sales are not a noteworthy way of saying something is good or bad because nothing is a noteworthy way of saying something is good or bad. Something being good or bad is purely subjective and up to each person, there is no true good things in this world or bad things because everyone will have a different opinion about everything.
Sales only show something is popular, and something being popular does not equal good.