case, casket
匵 centers on a container for storing or protecting something valuable: a case or casket. The two given meanings are close synonyms for a box-like receptacle.
匵 combines the enclosure 匚 (box) with 賣, which likely contributes a phonetic element. The character is rare and its exact historical development is uncertain.
The outer 匚 is a box, and inside is 賣 (sell). Imagine a merchant placing a precious item into a protective case before selling it.
For トク, picture a special case that is locked with a distinctive 'tock' sound: tock -> トク, as the lid closes securely on the casket.