Learn Farsi: بعضی از مشتری‌ها میوه‌ی تازه می‌خرند.

In Farsi, when you want to say 'some of the' before a plural noun, you use بعضی از (ba'zi az), as in بعضی از مشتری‌ها (some of the customers). You can also drop the از and just say بعضی مشتری‌ها (some customers), but adding از specifically gives the meaning of 'some of the'.

Why is there an extra ی on the end of میوه (mive) in میوه‌ی تازه?

That is the Ezafe linking the noun میوه (fruit) to the adjective تازه (fresh). Because میوه ends in a silent 'h' (ه), the Ezafe is written as an extra ی so it is pronounced 'mive-ye taze' instead of just attaching a vowel sound to a silent letter.

'Fresh fruit' is what they are buying. Why isn't there a را (ra) after it?

The object marker را (ra) is only used for specific, definite direct objects (like 'THE fresh fruit'). Here, the customers are buying 'fresh fruit' in general. Because it is an indefinite, general object, we do not use را.

What verb is می‌خرند (mikharand) from, and how is it formed?

It comes from the infinitive خریدن (kharidan), meaning 'to buy'. The می (mi) prefix shows that it is in the present tense (they buy / they are buying), the root is خر (khar), and the ند (and) ending matches the plural third-person subject (the customers).