sold a blacklisted phone

Purchased a Samsung S5 on [URL Removed] in December. Recently the phone stopped working(apparently the phone has been blacklisted) on the 11 March i called them and asked for the phone to be returned to [URL Removed] was told that the phone would be collected in three working days. It has been 12 working days and the phone hasn't been collected and every time i call them I'm being told that the courier will collect the device.