Learn French: Pourquoi est-ce que vous achetez ces oignons ?

ces is the plural demonstrative adjective, meaning "these" or "those". Since oignons (onions) is plural, you must use ces. The singular forms ce, cet, and cette are only used for singular nouns.

How does the pronunciation work when ces is next to oignons?

There is a mandatory sound link (called a liaison) between the two words. Because ces normally ends in a silent s but comes right before a vowel in oignons, that s wakes up and makes a "z" sound. You pronounce the two words together as "sez-oignons".

Why do we add est-ce que after Pourquoi?

est-ce que (literally "is it that") is a very common tool for asking questions in French. Putting it after a question word like Pourquoi (why) allows you to keep the subject and verb in their normal, everyday order (vous achetez) instead of having to flip them around.

The English translation says "are you buying". Why isn't there a word for "are" in the French sentence?

French doesn't have a separate "am/is/are doing" action form like English does. The single French present tense verb achetez covers both "buy" and "are buying". Trying to add a literal translation of "are" into this sentence would be a mistake!