I didn't mean to say most companies sell user data, but rather of the companies that sell user data, most are not straight forward about it.
Example: Onavo, Dolphin Browser, Opera Mini, Ask Toolbar
What I am emphasising is that while they could ask plainly, and state they are collecting your data, and then selling it to 3rd parties (even if in aggregate), they do not.
I would want users to be given a clear choice about what is happening with their data, and some companies have the integrity to do this.