Just to confirm, officially, that Mister Ed is correct, and to clarify where it comes from, the rules state that you can use one-shot Items directly from your hand or from your carried Items:
Quote:
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Originally Posted by The Rules, p. 2
One-shot items, such as potions, may be played directly
from your hand during combat. You can also use one-shot items that you already had in play
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Quote:
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Originally Posted by The Rules, p. 5
Any one-shot Item can by played during any combat,
whether you have it in your hand or on the table.
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Monster Enhancers and non-Item Treasures can't be carried, so you can't keep them in play, and there's no rule to allow for them to be kept in play.