no, they want to verify that its not stolen or IMEI blacklisted. a general rule is to never publicly provide your ESN/IMEI/MEID as there are people who can potentially clone them. but it isnt as much of a problem as it used to be. if someone was buying the phone, as in your case, i think they have every right to do their own homework to make sure the device is legit. when i bought my wife an iphone 4 a few years ago, i asked the seller for the ESN so i can verify with verizon it was clear. so yea, provide it to the seller.