Learn Farsi: چرا یک ساعت اصلی نمی‌خری؟

When a noun is described by an adjective, the indefinite marker -i attaches to the very end of the noun phrase, not to the noun itself. So, we start with sa'at-e asl (genuine watch), put yek at the front, and attach -i to the adjective, giving us yek sa'at-e asl-i.

Is the -i at the end of asli part of the word for 'authentic', or is it the indefinite marker?

In this specific phrase, it acts as the indefinite marker! The word for genuine or authentic is asl. By adding the indefinite -i to the end of the phrase, it becomes asl-i. (Note: Farsi does also have the distinct adjective asli, which usually translates to 'main' or 'primary').

Is there an Ezafe linking the noun and adjective here?

Yes, there is an Ezafe vowel connecting the noun to its adjective, making the pronunciation sa'at-e asl-i. Like most Ezafes, it is completely invisible in the written Persian script for ساعت اصلی.

Why does the verb nemi-khari translate to 'don't you buy' instead of 'aren't you buying'?

The Farsi present tense, built with the prefix mi- (or nemi- for the negative), covers both the simple present and the present continuous. While it can mean 'aren't you buying right now', making a general suggestion like 'Why don't you buy...?' uses the exact same grammatical form.