So to earn the trustee reward both parties have to agree. If one of both is only interested in a short term profit and is willing to exploit the other one than he/she can do it anyway: He or she can just use his newly generated coins after some time - exchange them all for the valuable coins of the other user and run away - create a new account.
This is why you really have to trust someone to not to do this - especially with totally new accounts. So if an account is not trusted by anyone yet you really DO NEED TO TRUST the person: so start only with friends and family. If the accounts is already trusted by a lot of (trustworthy) people the risk is much lower.