Clean ESN simply means that the phone is not currently tied to a phone number. Basically if you bought the phone at Verizon, activated it, and then went back to using your old phone the ESN is clean.
To ensure that it is clean, you can call verizon I believe and have them check. If you ordered the phone online, you need to activate on the phone line you ordered it for and then deactivate it by activating another phone on that number.
clean esn just means there arent any unpaid bills etc and that it's completely deactivated from your account. Asking for a clean ESN has become standard fare for craigslist and ebay purchases because people want to think they're being badasses by asking.
I think he means to ask you if the phone has a clean ESN. In other words, has the phone been reported lost or stolen. Give him your ESN # off your iPhone. He can call the wireless carrier and confirm it has a clean ESN
I agree with Munchkinsdad ... That's one of the security features of a CDMA phone, the ESN is attached to the device and the carrier can attach a hold on that ESN if you didn't pay your bill or reported the phone lost or stolen.