Several parts combine into one character.

The traditional 賣 is built ingeniously: 出 (to go out, on top) sitting above 買 (to buy, below). What was bought goes out again — that is selling. The character literally encodes the reverse-direction transaction. This pairs 賣 with 買 as one of the cleanest semantic couples in Chinese writing. All three CJK regions simplified the upper half differently: Japanese shinjitai 売, Mainland simplified 卖, while Korea retains the traditional 賣.

Korean reading "mae." 販賣 (panmae, sales), 賣買 (maemae, trade — literally "sell-buy"), 豫賣 (yemae, advance sale / pre-sale of tickets), 專賣 (jeonmae, monopoly), 賣場 (maejang, store / sales floor). Notice that the Korean word for "transaction" is itself the two characters 賣 + 買 lined up — Korean lexicalizes the entire sell-buy cycle in one compound.

Mandarin mài, 4th tone (simplified 卖). High-frequency: 卖东西 (mài dōngxi, to sell things), 售卖 (shòumài, to sell), 卖家 (màijiā, seller). Pairs with 买 (mǎi, to buy) — the two characters differ only by tone in Mandarin (mài 4th vs mǎi 3rd), which trips up beginners constantly.

Japanese on-reading バイ (bai) powers commercial vocabulary: 販売 (hanbai, sales), 商売 (shōbai, business / commerce), 売買 (baibai, buying and selling). Kun-reading うる (uru, to sell) — 売る is one of the most common verbs in everyday Japanese: 車を売る (sell a car), 売り切れ (urikire, sold out). The shop sign 売 means "for sale."

Memory aid: what comes 出 (out) of 買 (purchase) is what gets sold — selling is the reverse current of buying.