贝 depicts a cowrie shell — the earliest Chinese currency — and serves as the money/commerce radical. Every word about buying, selling, owing, or wealth uses 贝: 买 buy, 卖 sell, 财 wealth, 货 goods. 购 means to acquire by paying out.
勾 supplies the sound, with a tone shift gōu → gòu. The "hook" meaning has no role here — pure phonetic. In simplified 购 it replaced traditional 購's phonetic 冓 — a much more elaborate element.
