water plant
藚 refers to a type of water plant, specifically the arrowhead plant (Sagittaria). Its meaning is narrow and botanical.
藚 combines the grass radical 艹 with 賣, which likely contributes a phonetic element. The exact historical development is uncertain, but the grass radical indicates a plant.
The top grass radical 艹 sits above 賣, like a plant being sold at a market. Picture a water plant vendor selling arrowhead plants from a stall.
For ショク, imagine a shock of green water plants being sold at the market: shock -> ショク, and the plants are fresh and vibrant.