Well, worked it out.............................eventually!
Had to go to old xstreet, where all old Magic Boxes seen to be stored, and deleted them one by one. This then enabled me to delete items on Marketplace.
Why does Xstreet still exist anyway?
Same problem here, dozens of items are not, most definately not, in my Magic Box, but show up on marketplace. No matter how many times I reset box, then resync box, I am unable to delete these items