A.) Change the GT on the account that you purchased and put it on one of your own accounts. Problem with this is that you need to buy MSP.
B.) After buying the account change the Windows Live ID. This can only be done every 30 days so if they changed it before the trade you'll have to wait.
Changing the Live ID is probably the safest way to keep an account but if the GT is all you care about the Gamertag change is your best bet.
Oh then what you want to do is touble tap A on the dashboard and then press in this exact order B,A,B,B,B,B,RT,RB,A,B, Start, Start, B, X, Y and then A. This will secure both your runescape and xbox accounts to the maximum level of total private secrecy.