卖 (traditional 賣) is 买 (buy) with an extra part on top — the image of 'putting goods OUT' for sale. So 买 takes money in; 卖 puts goods out. The two are deliberately built as a matched pair.
卖 (sell) completes the trade pair with 买 (buy). Together 买卖 (mǎimài) means 'business / a deal' — '做买卖' is 'to do business'. 卖 also has a vivid extended sense: 卖关子 (to keep someone in suspense), 卖力 (to put in great effort, 'sell your strength'). Remember the tone: 卖 is mài, falling — the opposite of 买 (mǎi).
买 (buy) with goods piled on top — putting them OUT to SELL.
买 + 卖 = 买卖 (mǎimài), 'business'. 做买卖 is 'to do business / trade'. The buy-sell pair literally spells commerce.