This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
鑟 combines 金 (metal) with 賣, which likely contributes the sound. The exact historical development is uncertain.
The left side is metal 金, and the right side 賣 suggests selling. Picture a metal box used for selling goods, but the exact meaning is unclear.
For トク, imagine a metal box that makes a distinctive 'tock' sound when closed: tock -> トク.