Learn Hardcore Japanese: watasi ha sinpin yori tyuuko wo katte, ki ni iranakereba koukansimasu. - I buy used rather than new, and if I don’t like it I exchange it.

Pattern: bold A より B を V means bold do V to B rather than A. Here: bold 新品より中古を買う.
You put bold より after the thing you are comparing against (bold 新品), not the one you choose (bold 中古).
Variations:
- bold 新品よりも中古を買う (bold も adds emphasis)
- bold 新品より中古のほうを買う (bold のほう makes the preference explicit)

Is bold 中古 by itself okay, or should it be bold 中古品 or bold 中古のもの?

bold 中古 is a noun and is fine by itself in many contexts, especially shopping: bold 中古を買う.
You can also say bold 中古品 (more formal/shop-ish) or bold 中古のもの (spells out “used thing”).
As a modifier: bold 中古車, bold 中古の本, etc.

Why is bold 買って in the te-form? Could I say bold 買ってから instead?

The te-form links actions: bold 中古を買って、気に入らなければ… = buy used, and if you don’t like it, then…
bold てから emphasizes the sequence “after doing X”: bold 中古を買ってから、気に入らなければ… Both are fine; bold てから is a bit more explicit about “after.”
It’s normal to have a plain te-form before a polite final verb; politeness is set by the last verb (bold 交換します).

Who is the subject/object in bold 気に入らなければ? Do I need bold 私が or bold それが?

Japanese drops obvious elements. The experiencer is the speaker by default, and the item is the thing you just bought.
If you want to be explicit, you can say:
- bold それが気に入らなければ… (more common in writing)
- bold それを気に入らなければ… (colloquial; bold 気に入る allows both が and を for the liked item)

Why is bold 入る read as bold いる in bold 気に入る? How do I pronounce bold 気に入らなければ?

In the set phrase bold 気に入る, bold 入る is read bold いる. It’s pronounced bold きにいる.
bold 気に入らなければ is bold きにいらなければ.
Using bold はいる here is incorrect.

What’s the difference between bold 気に入る and bold 好き?

bold 好き describes a general, stable liking: bold その店が好きだ.
bold 気に入る is being pleased with something you encountered/obtained, often specific or new: bold この機能が気に入った; ongoing state: bold 気に入っている.
Both often take bold が for the liked thing; bold 気に入る can also take bold を.

Why use bold 〜なければ? How is it different from bold 〜なかったら, bold 〜なら, or bold 〜と?

bold 〜なければ: formal-ish, rule-like conditional; good for policies or general plans.
bold 〜なかったら: more colloquial, specific-time or if-then; very common in speech.
bold 〜なら: assumes a condition that someone brings up; often about suggestions or readiness.
bold 〜と: automatic/resultative; avoid when the main clause is a volitional act. bold 気に入らないと、交換します can sound too automatic; bold 〜ば or bold 〜たら is safer here.

Is bold 交換します the best verb? What about bold 返品します or bold 取り替えます? Do I need bold 〜てもらう?

bold 交換する: exchange for another item. From a customer’s viewpoint, natural as bold 交換してもらいます (have the store exchange it).
bold 返品する: return the item for a refund; no replacement.
bold 取り替える: replace or swap, often parts or small items; also used for exchanges, but stores commonly say bold 交換.
Politeness/agency: bold 交換します is fine as a simple statement of intent; bold 交換してもらいます is precise about the store performing the action for you.

Is the comma after bold 買って necessary?

No. It’s for readability. bold …買って気に入らなければ… is also fine. Commas in Japanese are flexible.

Does bold 交換します mean future or habit?

Japanese non-past covers both. Here it can mean a general policy or a plan for a specific situation; context decides.

Is bold 私は necessary? Is it natural?

Japanese often drops bold 私 when it’s obvious. Including bold 私は is fine in isolation or when contrasting with others.
Natural alternatives:
- bold 普段は新品より中古を買って、気に入らなければ交換します。
- bold 新品より中古を買って、気に入らなければ交換します。 (topic omitted)

Can I add bold のほう or bold は for emphasis in the comparison?