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.
luckly if it is on IOS 7, Ios 7 has a nice feature called find my iphone. You can log into your i cloud account and then go to the find my iphone. From there you can click a button to erase all content on the phone just with a click of a button. When the buyer then connects to wifi or has a internet connection signal from his phone provider it will manual wipe the phone clean.