watasi ha sinpin yori tyuuko wo katte, ki ni iranakereba koukansimasu.

は marks 私 as the topic: as for me or speaking for myself.

So 私は gives the sentence a broad frame:
As for me, I buy used rather than new, and if I don’t like it, I exchange it.

If you used が instead, it would sound more like you are specifically identifying me as the person who does this, rather than simply setting the topic.

Because 新品 is part of the comparison with より, while 中古 is the direct object of 買って.

• 新品より = rather than new items
• 中古を買って = buy used items

So the structure is basically:

• 新品より中古を買う
  buy used rather than new

Only 中古 gets を because it is the thing being bought in the sentence as stated.

Here より means rather than or more than in a comparison.

In this sentence:

• 新品より中古を買う = buy used rather than new

A native English speaker often first learns より in patterns like:

• AよりBのほうがいい = B is better than A

But it can also be used in shorter comparative expressions like this one, where the speaker is choosing one thing over another.

Yes, you can think of it as slightly compressed.

The full idea is something like:

• 新品を買うより、中古を買って…
• Rather than buying new, I buy used and...

Japanese often leaves out repeated material when it is obvious. Since 買う is already clear from 中古を買って, it does not need to be repeated after 新品.

So 新品より中古を買って is a natural compact way to say it.

買って is the て-form of 買う. Here it connects two parts of the sentence:

• 中古を買って
• 気に入らなければ交換します

This often works like and, but it can also imply sequence:

• I buy it, and if I don’t like it, I exchange it
• or After buying it, if I don’t like it, I exchange it

So the て-form is linking actions in a smooth, natural way.

気に入る is a set expression meaning to like, to be pleased with, or to suit one’s taste.

It does not break down neatly into natural English word-for-word, so it is best learned as a chunk:

• 気に入る = to like / to be fond of / to be satisfied with

A very important point:
気に入る is not the same kind of verb as English like.

In Japanese, the thing liked is often understood as the subject of 気に入る, even if that subject is omitted:

• このバッグが気に入った = I liked this bag
• literally closer to This bag pleased me

In your sentence, the thing bought is omitted because it is already understood.

気に入らなければ is the negative conditional form of 気に入る.

Breakdown:

• 気に入る = to like
• 気に入らない = not like
• 気に入らなければ = if I do not like it

So this part means:

• if I don’t like it

This form is fairly standard and neutral. It sounds a little more structured or formal than some alternatives.

For comparison:

• 気に入らなければ = if I don’t like it
• 気に入らないなら = if I don’t like it / if it turns out I don’t like it

Both are possible, but 気に入らなければ fits well in a polite sentence ending with 交換します.

The item being talked about is omitted.

Japanese often leaves out words that are obvious from context. Here, the understood meaning is something like:

• 中古を買って、もしその品物が気に入らなければ、その品物を交換します。

In natural English:

• I buy used, and if I don’t like it, I exchange it.

So the hidden object is the thing that was bought.

Its basic meaning is exchange.

So 交換します suggests swapping the item for another one. Depending on context, English might translate it as:

• exchange it
• replace it
• sometimes swap it

If the intended meaning is specifically return it for a refund, Japanese would more naturally use 返品します.

So:

• 交換します = exchange/swap
• 返品します = return
• 取り替えます = replace with another one

Your sentence is understandable with 交換します, but the most natural verb depends on the situation.

It can be either, because Japanese non-past forms often cover both.

• 買って
  • 交換します can describe a general habit:
    • I buy used rather than new, and if I don’t like it, I exchange it.
• It can also describe a plan or intention:
  • I’ll buy used rather than new, and if I don’t like it, I’ll exchange it.

The surrounding context tells you which reading is intended.

Because 新品 and 中古 can function directly as nouns.

They already mean:

• 新品 = a new item / new goods
• 中古 = a used item / secondhand goods

So 新品より中古を買う is fine.

You could make the nouns more explicit with something like:

• 新品のものより中古のものを買う

But that is longer and usually unnecessary unless you want extra clarity or emphasis.

Yes, it is understandable and mostly natural, but a native speaker might slightly adjust it depending on the intended nuance.

Possible points:

• 新品より中古を買って is fine as a compact comparison.
• 交換します is okay if you mean exchange it for another one.
• If you mean return it, 返品します would be more natural.

So the sentence works, but in real-life Japanese, the last verb is the part most likely to change depending on the exact situation.