Gmail puts context sensitive ads next to your mail. Some people say, it reads your mail so where's the privacy. Computers of course always read your mail, otherwise they won't be able to display it on the screen, in the sense that they go through it letter for letter, the same way as they do when to decide which ad to show. The real privacy issue is of course not so much Google, but more Yahoo and MSN.
Google seems to have the smarter ad display software, so it seems they know a lot about you, but everything happens just context based. There is no trail of previous searches or a list of clubs you are member of that influences this process. If you are a member of Yahoo!, it knows a lot about you and supposedly uses this information for all kinds of purposes. Some would argue it is an even trade, you sell your privacy in return for some services. You don't have to do it, you can also go somewhere else and pay for your hosting/email/discussion group or whatever it is that you get for free from Yahoo!
But is your privacy yours to sell? You cannot sell your vote, because we imagine that a society where people could sell their vote would be worse than a society where they could. Especially if we allow the reselling of privacy information, we will end up drammatically fast in a situation where there is no privacy at all. You sell your phone number once and everybody has it, because the cost of copying information is zero.
A better model might be one where I rent out my privacy information, for cash or for services. But as soon as I don't want the services anymore, the other party loses the control over the information. And if multiple companies want access to the information, they all have to pay for it. Also, people should be able to put their own price on their privacy. I really don't like to be called during dinner time about whether I want to refinance, but if they pay me 10 euro per conversation (with me saying, 'no thank you'), it might be different. Calling Bill Gates would be very expensive and calling some people would probably be almost free. The same for email and for the regular junk mail.
Of course a scheme like this can probably only be realized using a central clearing house for privacy, which in itself would be a thread to privacy, unless some complicated system with emcryption can be worked out.