I don't have advice to support the buyer in deleting your stuff, although you should change any and all passwords that were associated to that iPhone and its apps (if you remained logged in on any apps like twitter, email, etc).
Assuming the buyer is connected to a wifi network or a cell network, changing passwords will prompt them to enter the new password when they, for example, open the Mail app and it tries to refresh your inbox.
Not sure what else you can do.