Has the offer been made through the MBG?????  If not then I'd be wary of it, if you follow through with the MBG and ask eBay to step in you'll get your original postage and item price back and the seller will have to reimburse your return postage when they get the phone back (assuming you win the case)

Either way you decide to go, if you let the seller's courier pick it up be very certain you get the tracking from the courier before you let them have the phone.