I'm thinking of starting a business. It would be a website that would act much like a social-bookmarking site. It would be funded by selling data about websites and this data would be collected from users. The data would be in a completely anonymous form, for example: 10% of users who visited this URL also visited this URL.
What are the legal issues surrounding the sale of data of this kind?
You're asking a question which can have different answers based on your jurisdiction and/or geographic location (country of residence).
For any legal question, you can get general guidance from the Intarwebs, but you should also have a real lawyer of your own who can give you more precise answers and hopefully back you up if you follow his advice and later run into issues.
The above being said, if you are in the US and you have some click-through disclosure that tells users statistics about their usage patterns will be sold to 3rd parties you should be free of liability. This is assuming you are not providing any information or data that could be used to personally identify a given individual and/or put anyone at risk of exposure.
This is essentially what Netflix did with the anonymized user data they released as the basis for the Netflix Challenge a couple of years ago.
I agree with the post above.