Mnemonic for differentiating 买 and 卖
Here's an image shows the evolution of 賣/卖.



Pay attention to the picture (B), it appeared at the Qin dynasty, and take the 買/买 as its phonogram part, so 賣/卖 has the same pronunciation with 買/买, and take the word 出(out) at the top of the word, which means sell out sth to get money. And at Han dynasty, 出 became 士 (picture (C)), and then at Tang dynasty's cursive script(picture (E)), it was simplified as 卖 further, and at last 卖 is adopted as the simplified Chinese word of 賣.

Hope it can help you to remember the difference between 买 and 卖.

Source: [字源谈趣] Author: 陈政