Important Notice: Account sharing and selling is against the Terms of Use and is bannable

You do not own your account or any content in it. All of it belongs to Riot and you are merely "renting" it.

You 'sign' a legal agreement called the Terms of Use every time they patch the game which states in no uncertain terms that you can not share or sell your account.

If an account you buy is banned (as expected), it is not on Riot to reimburse you or give you another account, because the transaction was illegal and not recognized by them. Take it up with the scam artist that fooled you.

Account sharing is not as serious, but in the case of MMR Boosting it is just as harmful to the game. Sharing your account info puts it at risk of theft, where bannable actions may be taken; if you shared your info, you are responsible for those actions.

If your account was stolen and you did nothing against the ToU or EULA, then you can submit a Support Ticket and Player Support may be able to help you. For more information on account security, go to the support page here.