I just bought a mobile phone on ebay and the imei number on the mobile phone doesn't match that it says on the box, should I be concerned about this?
also if i get a fault on my phone in the future will nokia check it if i say i got from ebay, the seller didn't send me a receipt for it. on the ad he says he got it brand new,

If they say they got it from new, Id be very concerned. Although its not unknown for sellers to buy boxes for phones that are not new, and buy a cheap case which makes it look brand new.

Id be asking for some money back.

The simple, benign explanations:

1: The phone had to be repaired and the system board changed -> new IMEI

2: The box is for some other phone (the seller had more than one and stuck the phone in the wrong box)

Check whether the label under the battery reports the same IMEI as *#06#