A Treasure card that says “Usable once only" is often called a “one-shot" Treasure. Most of these are used during combat to strengthen the munchkins or the monsters, and may be played from your hand or from the table. Some have other effects, however, so read the card carefully!
Discard these cards as soon as the combat is over or their effect is resolved.
One-shot Items with a Gold Piece value may be sold for levels, just like other Items.