As a general rule, it's often a box that has been slightly damaged (during transport, for example) to the point where it can no longer really be resold as 'new'. As a result, it's resold as a controlled return, but the contents are checked and often the bags aren't even opened.

I'm sorry I can't answer the questions yet! The Gringotts Wizard Bank will not be given to a Family member until Christmas 2025! The pack hasn't been opened yet!
And - it's not for sale at the moment!

Yes, the bags are numbered, some of the stickers are already attached, some you have to stick on yourself. The stickers are also numbered.