My recommendation here is to:
1. Make sure any accounts that are tied with it are in reach, or disabled.
2. Be prepared to watch out for possible negative trust if you've bought an account as people like to discourage it
3. Be on your best behavior. Don't give account purchasing a bad name.
People can't negative account just because he bought it from another forum member, they can discourage, but it's not against the rules. Sometimes this happen when the account was already negatived when bought, when the account buyer use it to make spam posts or try to scam another members.
Don't worry. I don't think most people would actually purchase a negative rated account anyway. Most sold accounts are fresh accounts with no markings on them.