Bellaboo2

hi, the rear seatbelt fired on my b class, I've removed them and purchased some new ones (eBay). I have fitted them back in position but the belt will not release. Ive replaced belts in the past on other makes of car with no problem. The seller said new belts are "locked" until fitted, but that makes no sense to me as the only electrical connection is the 2 pin plug that fires the ball bearings into the mechanism if the eco triggers it... Any advise would be welcomed, thanks Mark