买
mǎi
Even the bill gets bought — what 买单 really says
The picture The money in your hand crosses over, and in its place the goods cross back to you. 买 sends money out and brings goods in; 卖 sends goods out and brings money in.
Where the picture shows up in 买
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goods cross to you
我去买点水果。
wǒ qù mǎi diǎn shuǐguǒ
I'm going to buy some fruit.
A note changes hands at the market and a bag of fruit comes back
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bought, then passed on
这个是给你买的。
zhège shì gěi nǐ mǎi de
I bought this for you.
The just-paid-for package is set straight into someone else's hands
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the bill crosses to you
服务员，买单。
fúwùyuán, mǎidān
Excuse me — the bill, please.
At the end of the meal a raised hand draws the bill over to your side of the table
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already crossed over
票已经买好了。
piào yǐjīng mǎi hǎo le
The tickets are already bought.
Two printed tickets pulled out of a pocket to show they're in hand
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nothing left to cross
这个型号买不到了。
zhège xínghào mǎi bu dào le
This model can't be bought anymore.
A hand holding cash in front of a shelf that has nothing left on it
All of it in one picture
买 is one picture: money and goods trading places. Let fruit cross over to you and it's doing the shopping; hand what crossed over to someone else and it's buying them a gift. Let the bill cross over to you and you're the one paying; have the tickets already crossed over and the booking is done. Hold out money with nothing left to cross back and it becomes unavailable.