You cannot buy or sell twitter users!

Check this twitter rules.

Although it is not a legal matter, it is now allowed to buy or sell twitter usernames.

"Selling usernames: You may not buy or sell Twitter usernames."

see this twitter rules under the section of 'Abuse and Spam' at https://support.twitter.com/entries/18311

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Report if you see somebody trying to sell twitter followers! - https://support.twitter.com/forms