Learn Farsi: هیچ‌کس ربع سهمش را نفروخت.

Yes! Fractions like rob' (a quarter) and nim (half) act like regular nouns in Farsi. To say a quarter of something, you connect the fraction to the following noun using an ezafe (the hidden 'e' sound). Here, it is pronounced rob'-e sahm-ash.

Since hich-kas already means nobody, why is the verb nafrukht (didn't sell) negative?

Farsi requires double negatives! Whenever you start a sentence with a negative word like hich-kas (nobody) or hich-chiz (nothing), the main verb must also be negative. A literal translation would be 'Nobody didn't sell', but in English we just say 'Nobody sold'.

Why is the object marker ra placed after sahm-ash instead of right after rob'?

The word ra must come at the very end of the entire phrase that acts as the direct object. The whole phrase 'a quarter of his share' (rob'-e sahm-ash) is the object being sold. Because the possessive ending -ash (his) makes it a specific share, the whole group of words gets capped off with ra.

Does rob' work the same way when telling time?

Yes! Just like in English, rob' is used both for dividing physical things (like a share) and for telling time. If you want to say 'a quarter past one', you say yek o rob'.