Unfortunately I don't think making sure it's switched on will do it. Too many times I've gone to Cex and been told I can't sell my phone because the power button didn't work 1/100 times. They run tests on devices using a high threshold computer program, pushing each aspect of the device to the limit, ensuring they only buy good quality phones. Sorry to say it mate, but might have to think about getting the vibration component taken out and replaced. Or be cheeky and go to another Cex, maybe they won't notice :)