Does this sentence also mean 'Do you believe it?', as it does in English?
Yes kind of, in slang
So what are the root letters of "קונים", bit confused with this one...
Fun thing I have realized - that long vowel ו in the second position is almost never a root letter, and almost always indicates you’re looking at the verb version of the word.
How would one ask, "are you buying these"?
אתם קונים האלה?
You should always use ״את״ before a direct object
... before a definite direct object.
אתם קונים את אלה?
Why את זה?
As opposed to what? Depending on what you have in mind, your question may already have been answered on this page.