Playing items that are not usable at this time to the table.

[inglijoh]

Hi All
I am a very new player but i am loving the game. Can i place an item (big or small) on the table at the start of my turn that i am not using but want to keep in inventory. So i would place them on the table sideways. The other part to this question assuming that i can do that is when can i exchange a weapon that is on the board as inventory for one that is in play and being used.
Thanks inglijoh