Keep in mind that a list of valid member e-mails is one of a website's most valuable assets.
 
But, if a site was selling the member's e-mail addresses, would they tell anyone? Would they even tell other people in the company? What would the marketing department think if their anti-spam advertisements were becoming worthless because someone in the sales department was selling e-mails to the very people that the marketing people told customers they were being protected from?
 
So, you could ask Yahoo and Hotmail (for example) if they sell their addresses and they'll always tell you "no"... even though the person who responds may not even know that the sales department is doing.
However, it's tough (and wrong) to accuse a site of selling e-mail addresses without definite proof.

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