The community seems to be split on this one.

Are websites such as Amazon,  Gamestop, etc a good indicator for sales of a product in a manner that if it is selling good at the website it is doing good at retail worldwide?

Vote.

there is more to it than that. They are sometimes our only source of info, in those cases what else can you use? But  as far as compared to lets say vgc numbers probably not. They will not be as accurate, and a lot of times there are no hard numbers, just positions relative to other items or if they are sold out or not.so i would say they are only of use in waiting for more concrete numbers from some sort of tracking site.

You need to factor in the Amazon special deals (like future video games credits) when looking at the relative sales.  Not all games get them at Amazon, so those will appear to be selling less.