This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
蕒 combines the grass radical 艹 with 買, but the exact historical development and original meaning are uncertain.
The grass radical 艹 sits above 買, but without a clear meaning, it is hard to form a reliable mnemonic. Perhaps imagine buying a mysterious plant, but treat this as a placeholder.
For バイ, imagine buying a rare plant: buy -> バイ, and the grass radical 艹 hints at a plant.