Reading through the description of the Crystal Ball of True Seeing and spell I'm a little curious. Do you need to know the spell Scrying in order to cast it? Also, does it consume a spell slot when casting? For example, would a level 7 wizard, which is not high enough level yet to cast the spell, be able to use this item or not?
The Crystal Ball is a very rare item, this makes it powerful in the first place.
The character needs to first be attuned to the magic item in order to use it and from my understanding (on the DMG's page 159), while touching it you're able to cast scrying (with a minimum save DC of 17).
This doesn't come with any limits, you can use this as many times as you want during a day and don't need to be a spellcaster or have spell slots in order to use this (again: all you have to do is be attuned to it), you can read about this on the DMG page 141, Activating an Item: Spells.
The answer is in the DMG (p. 141)
Some magic items allow the user to cast a spell from the item. The spell is cast at the lowest possible spell level, doesn't expend any of the user's spell slots, and requires no components, unless the item's description says otherwise.
The crystal ball doesn't "say otherwise" so you don't need to know the spell and you don't need to have spell slots.
Spell Scrolls, for example, do "say otherwise".