买 (mǎi, 3rd tone, to buy) and 卖 (mài, 4th tone, to sell) look and sound similar but are opposites. The extra stroke on top of 卖 can be remembered as 'putting something out for sale.'
The single top hook is the 乛 form of the 乙 radical and is what xinhua files 买 under. In the traditional 買 the top piece was 罒 (net); the 1956 reform compressed it to this hook stroke, which is now the official indexing component.
Below the hook sits the standalone char 头 head. Strictly it is the bottom-portion 貝 (shell-money) of the old 買 redrawn as 头 during simplification; treat it as the body that carries the buying-with-shells imagery into the modern form.
