Learn Farsi: هیچ‌کس هفتمین چاپ را نخرید.

This sentence uses the pre-nominal emphatic form of ordinal numbers. By adding -omin to the number (instead of just -om), it goes before the noun and doesn't take an Ezafe. If we used the standard post-nominal form, it would be chap-e haftom. Both mean the seventh edition, but haftomin puts slightly more emphasis on the number.

Could I write haftomin-e chap with an Ezafe?

No, you never use an Ezafe when using the -omin ending. The number connects directly to the noun without any linking vowel. You only use the Ezafe with the -om ending, which always goes after the noun (like chap-e haftom).

Why is the verb negative (nakharid) if the English translation is positive ('Nobody bought')?

Farsi uses a mandatory double negative with words like hich-kas (nobody) or hich-chiz (nothing). When you use hich-kas as the subject, the verb must be made negative. Literally, the Farsi sentence says Nobody didn't buy the seventh edition.

Why is the direct object marker ra used after chap?

ra marks a specific direct object. Because we are talking about a specific, identifiable item—the seventh edition, not just any random edition—it is definite. Therefore, we must add ra right after the entire noun phrase haftomin chap.