Learn Farsi: بعضی از مشتری‌ها شیر نمی‌خرند.

بعضی means "some". When you want to say "some of the" before a noun, you use بعضی از (ba'zi az) followed by a plural noun, like مشتری‌ها (customers). You could also use the Ezafe connector instead (بعضیِ مشتری‌ها), which means exactly the same thing.

Do nouns always have to be plural after بعضی از (some of)?

Yes! Unlike numbers in Farsi (which take singular nouns), بعضی از must be followed by a plural noun. That is why we use مشتری‌ها (customers) with the plural suffix ها, rather than just the singular مشتری.

شیر (milk) is the direct object of the verb. Why isn't it followed by را (ra)?

The marker را is only used for specific direct objects (like "the milk" or "that milk"). Here, the customers are just not buying milk in general. Because it is a general, non-specific noun in this context, we leave را out.

How is the verb نمی‌خرند (they do not buy) built?

The infinitive is خریدن (kharidan - to buy), and its present stem is خر (khar). To make this negative present tense verb, we combine the negative prefix نـ (ne-), the continuous prefix می (mi-), the stem خر, and the "they" ending ـند (-and).