This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
珻 combines 王 (jade) with 每, which likely contributes the sound. The exact historical development is uncertain.
The left side 王 suggests jade or a precious stone, and the right side 每 is a common phonetic component. Without a clear meaning, picture a rare, uncut gem whose name is lost.
For バイ, imagine buying a mysterious jade stone: buy -> バイ, and the 王 component hints at its gem-like nature.