Sales is not a horrible indicator at all. It shouldn't be the sole indicator, but to say it's bad is pretty stupid. Queen sold over 300 million records world wide, the Bee Gees have sold over 200 million, The Beach Boys about 50 million. All great bands.
I personally think that if The Beach Boys had chosen different subject matter, i.e. not surfing, to write about, they probably would have done a lot better sales wise. Their biggest problem though, was alienating their entire fan base by releasing Smiley Smile, which sucked, after cancelling Smile.
They never really recovered after that, despite recording some really decent albums, and songs, like the Sunflower album (which did horribly). However, people just weren't interested in The Beach Boys post 1967, which I think is very sad. Personally, I think they were one of these most under appreciated and under rated bands of the 70's.