The distinction between `购买 (gòumǎi)` and `买 (mǎi)` highlights the importance of formality levels in the Chinese language. Chinese has a clearer distinction between written language (`书面语 shūmiànyǔ`) and spoken language (`口语 kǒuyǔ`) than modern English does. `购买` belongs firmly in the category of written or formal language. Using it correctly shows a higher level of linguistic sophistication. A great comparison in Western culture is the difference between “to purchase” and “to buy.” You might tell a friend, “I'm going to buy some milk,” but a company's financial report would state, “The company will purchase new equipment.” The nuance is almost identical in Chinese. Using `购买` for buying milk would sound just as strange and overly formal as using “purchase” in that English sentence. Respecting this distinction is key to sounding natural.
`购买` is extremely common in specific, modern contexts.
It is almost never used in casual, spoken conversation for everyday items. For groceries, coffee, clothes in a shop, you should always use `买 (mǎi)`.