There are many instanes when they display Baseball Cards, maps & letters on a black felt(?) board. The items are always secured with the use of thumb-tacs. Aren't thumb-tacs a no-no?
Yes it appears they are using magnets however those could possibly leave an impression on aged, fragile paper and the like even when only used for a short time. I'm often stunned by how carelessly some of the appraisers handle these items that are worth thousands such as tipping glass at angles on a small table (and holding it with just one hand) to show maker's identification marks and the like or using a pointy stick to point out something which could possibly knock the item or pierce it if they happen to lose their grasp or slipped a bit. There must be some stories of appraisers accidentally destroying the item on the show (I remember one episode when an appraiser was talking to an owner when you heard a very loud crash of broken glass or pottery in the background though that could have been broken by one of the people who brought the item to the show).