sell
売 centers on the act of selling: transferring goods or services in exchange for money.
sale; offering for sale; putting on sale; putting on the market; release (for sale)
sale; sales; selling; marketing
to sell
to sell (well)
sale; selling
amount sold; sales; proceeds; takings; turnover
selling off; disposal by sale; sale
trade; business; commerce
trade; buying and selling; trafficking (e.g. of humans, arms, drugs); dealing
resale
Yomiuri (newspaper, etc. group)
prostitution
retail
selling out; being sold out
sold-out
selling area; counter; section; department; sales floor
wholesale; wholesaling
stand; stall; booth; kiosk; store
auction
article for sale; goods for sale; offering
(bargain) sale
popular line (of products); best-selling line; hot sellers
monopoly; monopolization; exclusive sales rights
bargain sale; selling cheaply; selling at a low price
売 is the modern simplified form of 賣. The older form 賣 combines 出 (go out) and 買 (buy), suggesting the idea of selling as the opposite of buying. The modern simplification replaces the lower part with a stylized shape.
The top 士 looks like a merchant's stall, and the lower part resembles legs walking away with goods. Picture a merchant selling items from a stall as customers leave with their purchases.
For バイ, imagine a busy market where you buy and sell: buy -> バイ, and the merchant sells you something.