Where the shape comes from
A cowrie shell: the box is the shell with its slit, and the two legs below are the feelers of the animal. Cowries were the earliest currency, so the radical marks value and transaction — 贵, 货, 财, and the traditional 買 and 賣, whose simplified forms 买 and 卖 have lost the shell.
汉字 filed under 貝
No published character on HSK Guide is filed under this radical yet. 汉字库.