Money and shopping

买 (mǎi) is to buy. 卖 (mài) is to sell. Third tone and fourth tone, identical otherwise.

This is the example every Mandarin teacher reaches for, and it deserves its reputation. The two words are opposites, they occur in exactly the same sentences, and context frequently does NOT disambiguate them. 我要买 and 我要卖 are both perfectly sensible things to say in a shop and they mean opposite things.

If you have been treating tones as decoration, this is the pair that ends that. It is also the pair worth over-practising, because a third tone before a fourth tone undergoes a small change in real speech and the distinction gets slipperier than the textbook suggests.

The characters help: 卖 has an extra stroke on top, and the mnemonic that selling is buying plus something extra is not etymologically sound but does work.

块 is what people actually say

The formal currency word is 元 (yuán). The word people use in shops is 块 (kuài), which means a lump or a piece.

三块 is three yuan. You will hear 块 constantly and 元 mostly in writing and on price tags, in the same way English speakers say quid and write pounds.

花 spends and flowers

花 (huā) is a flower. 花 is also to spend, of money and of time: 花钱, 花时间.

The connection is scattering or spreading out, and it is the same instinct as English blow on money, if less unkind.

有钱 is a state

有钱 (yǒu qián) is rich, literally to have money. Mandarin often builds adjectives this way, with 有 plus a noun: 有名 is famous, have-name. 有用 is useful, have-use.

This is worth adding to your stock of guessable patterns, alongside the compounding you met with 听说.

客人 and 客户

客人 (kè rén) is a guest, and also a customer in a shop or restaurant.

客户 (kè hù) is a client in a business sense, an account. A shop has 客人; a consultancy has 客户.