买 (mǎi): To buy, to purchase. This character can be pictographically imagined as a net (网) used to acquire or “catch” goods.
卖 (mài): To sell. This character looks very similar to `买` but has a “plus” sign (十) on top. You can think of this as a person (士, an ancient radical for scholar/gentleman) standing up to offer goods for sale. The different tone is crucial for distinguishing “buy” from “sell.”
Putting them together, 买卖 (mǎimài) “buy-sell,” logically and transparently means “trade” or “business.” It represents the complete, two-way flow of a commercial exchange.