Okay I'm looking to buy an iPhone 5S and I found an advert for a refurbished model. Looked online to find risks associated with this and it mentioned warranty and IMEI number as ways to check if the phone is legitimate. So the warrantee is for three months - seems reasonable to me. But when I asked about the imei number they said they could not provide this in case I did not buy the phone.

Now I'm clueless in this area - I heard about imei numbers for the first time today. Is this number not supposed to be given out to potential buyers or is it a sign of suspicion that the seller is holding it back? Thanks.