It's definitely illegal...
Read this and see how:
[cantpostlinks]
~ Quote:
The company said pages that have been abiding by its rules should expect no more than a 1% dip in Likes. Even Zynga's big drop represented only roughly .3% of the more than 65 million people who like that poker page.
Facebook's purge targets inauthentic Likes created a number of ways. Some are caused by malware. Others happen when a user is deceived into "liking" something they may have not intended to like.
In its post, Facebook said it has never allowed Likes to be sold, but that it has caught third-party vendors trying to use malware and other forms of deception to do just that. Similar problems have plagued Twitter, where automated systems have tried to inflate users' follower counts.
"While we have always had dedicated protections against each of these threats on Facebook, these improved systems have been specifically configured to identify and take action against suspicious Likes," Facebook said.
~Unquote
Abiding by rules: following rules. If you don't follow their rules, they can take any action, includes legal actions. They have mountains of money, they have top lawyers who would love to make you an example for others...
Other are doing it, so what... if you are scared of getting caught in the net then don't do it.
P.S.:
Others may not get caught even if they sell a million fake likes and followers, you could get caught even if you don't sell 10! It depends on how lucky or unlucky you are, and how much angry they are on all general sellers of fake likes and followers. It can happen tomorrow or in a month or a year. Why take chances, especially when you're scared?