When it comes to buying twitter followers you need to understand that most are not real accounts and get canceled by twitter.
There are other people who will just add your twitter id to some online system that people will use to get followers. The way it works is that people may get credits for following you, but then they unfollow you once they get their credits.
The one thing you are going to learn, like I did the hard way, is that the only way to get followers that stay with you is to do it the old fashioned way.
Even if someone gets you followers that stick, they are probably still fake accounts, so they will not be clicking on links or retweeting your tweets.