The Account trust scam is a scam that involves a scammer stealing another player's account by building trust with them. Some low-level players may befriend a higher-levelled, rich or member character. They may be friends for a while when the lower level player asks if the victim if he/she can play on the higher-levelled account. They exchange passwords. The victim plays the higher level charater's account for around a while, and then changes the passwords with both accounts. If the victim doesn't have account recovery questions or a registered e-mail address, then he/she may lose his/her account forever. This directly breaks the 'account sharing' and 'Scamming' rules under honour.
It is important to remember that according to the Jagex Terms & Conditions, each RuneScape account may be used by only one user throughout the lifetime of the account. As a result, sharing accounts is prohibited under the Rules of Conduct, by the 'Account sharing' rule under Honour.