If you fill all of the item slots with items being sold, they will stay in there until sold. I recommend completely making sure you sell them all, because if you don't, and the items decrease or rise outside of the G.E market vlaue that you had it in there, you may have a problem selling the item. I guess it would only be if it decreases, so it's a risk to keep it in there
If you are buying items and they rise you won't get them.