It's illegal, and usually ends up with one side getting scammed of either money or the account itself. It's not encouraged to try, but there have been cases of successful sales. Do not try it unless you can do it privately and with someone you can trust
You can find a user to get to their profile if you start a post on activity then in the post type @ then start to type their name and it will come up to tag them. Then go to the tag and then you will see their profile and can pm or add them :)
I pointed out the rule so that people would know it. I did not look to see how many times he had posted. I just want more people to know this rule in case people missed it as it is way down at the bottom.