whats the problem
only downside to them can be the price,or availability
theres more chance of a second hand one being safer than a new one .quite a lot of them have been recalled as faulty last year . that includes wylex volex and crabtree . ive just fitted a new mk double pole rccd and it was tripping at 15ma instead of 30ma . and got replaced so there u have it nothing wrong with using second hand breakers i have loads of them from upgrading to larger db si wouldnt ever fit a used mcb,not if i got it free