you said you were sure he was sharing?
if the trader told you something like "i traded the acct and it was locked due to the trade."
then i would say it was completely his fault and you didnt need to do anything.
however, keep in mind when you trade there is often a lot of transfering/renaming, etc. and this can lead to locks due to suspicious activity.
if the account is locked because he was doing transfers or name/faction changes etc after you traded him, i would say that you are responsible to AT LEAST do everything in your power to help him (ie: id recover the acct if you are OO, provide them with an ID you have from the OO, provide CD keys, etc.).
all in all i would say even then its not your responsibility persay, but keep in mind that this is a trading COMMUNITY and if you arent willing to help ppl out with problems they have whether its our account their account, or a random person asking for help, then you arent going to have a good reputation or experience in that community.