Mystery box
A mystery box is a box sold with a published range of contents instead of a fixed list. The exact cards are named only when the box is opened.
In more detail
A mystery box is sold with a stated range of contents and not a fixed list. The seller may state the kinds of cards it can hold, a lowest and a highest value, or a guaranteed card, and the exact cards are named only when the box is opened.
The useful question is what the seller states is inside. A seller who publishes a checklist, a value range or a guaranteed item gives you something to check afterward; a seller who says only that it is a mystery gives you nothing to hold them to.
A mystery box is not factory-sealed product from a maker. The seller picks the cards that go in, so judge it by the seller's record and the stated contents, and compare the price with what the stated range is worth.
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