买 (traditional 買) is a net (网) over a cowrie shell (贝). Cowrie shells were ancient China's money — so 'netting in money/goods' = to BUY. Almost every character about wealth or trade contains 贝, the money shell.
买 (buy) and 卖 (sell, next) are a famous learner trap: they look almost identical (卖 is 买 with a lid on top) AND sound almost identical — 买 is mǎi (3rd tone, dipping) and 卖 is mài (4th tone, falling). One tone apart, opposite meaning. Get the tone wrong and you've flipped buying into selling! 买东西 = to go shopping ('buy things').
A net (网) scooping up money-shells (贝) — to BUY.
买 (mǎi, buy) and 卖 (mài, sell) differ by ONE tone — 3rd vs 4th. Mix them up and you've swapped buying for selling. The characters are mirror-twins too: 卖 is 买 with a 'lid' on top (putting goods out).