卖

Top 十 - the indexing radical of 卖 (Kangxi #24). Visually a cross, but historically a stylized residue of 出 'go out': in the traditional 賣, goods 'go out' to the buyer. The 1956 simplification flattened that 出 down to this two-stroke 十 shape. Pedagogically: 卖 = the act-of-selling that lets goods 'cross out' to a 买 buyer below.

Bottom 买 'buy' - supplies the sound (mǎi → mài, just a tone shift) and creates the most elegant pair in the script: 买 = buy, 卖 = sell. Adding the 十 cap on top of 买 turns 'buy' into 'sell' - goods going out instead of coming in. Both characters share strokes 2-7 here as a visual fingerprint of the buy/sell relationship.