It has been explained many times before in these auctions threads .. But for those who are not aware.
These items are from peoples private collections. The are things MJ either gave away, gave as gifts or auctioned away in charity events etc etc .. They are not part of Michael personal belongings that were in his possesion , his home or his storage. They no longer belong to Michael or his Estate. The were given away or abanndoned in hotels by MJ years ago.
Many of MJ's clothes especially costumes, he had more then one piece of the same item made. He autographed and gave many away for charity auctions or as gifts. Items that are autographed show he gave them away for charity or maybe as payment for a favor or as a gift.
The item being auctioned belong to the people in possession of them. I hope that helps ..
But keep the faith
Eventually many of these items will end up in a MJ museum. People will donate them with a plaque stating donated by .. or loaned by .. as most collecters would want their treasures to be on public display rather then hidden in their closets where no one can see them.