2) Clearly state in your TOS that by using your website that the user allows you to access and sell their personal information.
Those two steps should cover your butt legally. (I am not a lawyer and as always you should consult an actual attorney for a professional opinion).
The following steps are good netiquette and should help avoid complaints and controversies.
3) Put a copy of your TOS in your registration page. Don't just link to it, put the actual content there so it is impossible to miss/avoid. (I put it in a textarea so it does't take up a lot of space and scrolls but this can be accomplished other ways as well).
5) Allow members to opt out of your service and delete all of their information. This includes no longer selling their information to third parties.
6) If you are in the US, become a member of the Better Business Bureau.