Character difference between 'buy' and 'sell'
The original form of the top part of 賣 is 出.
《說文》

作。出物貨也。从出从買。

出物貨也：
出：拿出 take out
物：物品 goods
貨：交易 to trade

買
《說文》

作，市也。

市：做買賣。to trade
「買」本指商品交易，兼有「買、賣」二義，後來專指交易中的買入。
買 refers to the commodity trading activities, both &quot;to buy and to sell&quot;, and later refers specifically to purchase transactions.
source: 《康熙字典》《字源》