Translation:I will not buy chairs.
Someone correct me if I am wrong.
は is the topic like you said, so we infer 私が so I am the subject, and this approximately means: Chairs do not get bought by me. (that is not something I do)
を however is the object of the verb (similar to accusative case in English), in this case we infer 私は, so with を the sentence would be: I am not buying chairs. (at the moment, I can pick some up later)
I disagree with you. Here, using は means you may not buy chair, but maybe something else (other furniture). If you use を it's just fact that could mean anything without context : I don't buy chair.
は is either replacing the subject mark (が) or the object mark (を) when it is alone, emphasizing the word before.
By the way I wouldn't use the passive form if I were you, because there is one in japanese... To make a verb in passive form, using dictionnary form, transform the う in あ for the first group (except verb finishing by う, it becomes わ) then add れる; for the second group just add られる ; last group exception : される/こられる.
Example : 使う => 使われる
食べる => 食べられる
This one specifically has a few ways to translate it, which is why you see people saying "I will not buy a chair" or "I will not buy chairs" and a few other variations.
Usually, if the amount matters, you'll see a counter following a number. For example: いすが二-つは買いません。 I will not buy two chairs makes the number clear. (might have messed up particles, sorry)
Because that is how negative statements are generally formed in Japanese; there is a more complex answer, but it gets into the nature of the particle "は” and the fact that it has several distinct uses, none of which map well onto English grammar. If you're still curious, look up "using は for contrast" or something similar. Essentially, the formation is something like "I do not buy chairs [but I might buy something else]".
"Don't buy chairs." is imperative, giving an order to someone while 「いすがかいません。」 is declarative, describing whether someone (Implicitly the speaker unless context says otherwise) will buy chairs. The English passive voice is closer but still not an exact match "Chairs will not be bought."
"I will not buy chair" does not make sense. You need 'a' or 'the' in front of 'chair'. forum.duolingo.com/comment/23148160$comment_id=30235046