"in mint condition
if something is in mint condition, it looks as if it is new
The mint is a place where new coins are made.
There's an ad here for a 1974 Volkswagen Beetle. It's dark blue and in mint condition, apparently."
"mint" (or MIB) as an adjective describes things (especially for collectible things such as coins and stamps) which have not been used even though they may be old. So, something that is MIB looks absolutely new.
Example: If you're a collector of Beetles' records, you'd probably be willing to pay a great deal for a mint copy/ MIB of their early recordings.
I suggest that you do a quick Google search on"Mint in Box" for a multitude of examples using this term to describe a multitude of products (especially collectibles)