kounyuusya ga haraimasita.

It is read:

こうにゅうしゃ が はらいました。

• 購入者 = こうにゅうしゃ
• が = が
• 払いました = はらいました

A natural Romanization would be Kōnyūsha ga haraimashita.

購入者 is a noun meaning purchaser or buyer.

It is made of:

• 購入 = purchase
• 者 = person

So literally, it is the person who purchased.

This is a fairly formal or business-like word.

が marks the grammatical subject of the sentence.

So in 購入者が払いました, 購入者 is the one who performed the action of paying.

A common way to think about it is:

• 購入者が = the buyer is the one who...
• 払いました = paid

In many contexts, が can also add a sense of identifying or emphasizing the subject.

You often could use は, but the nuance changes.

• 購入者が払いました。
  Focuses on who paid. It can sound like The buyer paid or It was the buyer who paid.

• 購入者は払いました。
  Makes 購入者 the topic: As for the buyer, they paid.

So が is more natural if the important information is the identity of the person who paid.

払いました is the polite past affirmative form of the verb 払う, which means to pay.

Breakdown:

• dictionary form: 払う
• polite present/future: 払います
• polite past: 払いました

So 払いました means paid in polite speech.

Japanese often leaves out information that is understood from context.

So 購入者が払いました。 does not explicitly say paid what?, but listeners will usually understand it from the situation.

Possible implied objects could be:

• 代金 = the price
• お金 = money
• 料金 = fee
• 請求額 = bill amount

If you want to state it clearly, you could say:

• 購入者が代金を払いました。
  The buyer paid the price.

Japanese normally puts the main verb at the end of the sentence.

That means a basic pattern is often:

subject + object + verb

or, if there is no stated object:

subject + verb

So:

• 購入者が = subject
• 払いました = verb

This is completely normal Japanese word order.

Yes, it sounds somewhat formal.

There are two main reasons:

• 購入者 is a formal word for buyer/purchaser
• 払いました is in polite -ます form

This kind of sentence would fit well in:

• business Japanese
• customer service
• reports
• written explanations

In everyday conversation, people might use a less formal expression depending on context.

No. Japanese nouns usually do not show singular vs. plural unless the context makes it clear.

So 購入者 could mean:

• the buyer
• the buyers

Context tells you which is meant.

If the speaker needed to make plurality clear, they would usually do it in some other way, such as adding more information.

Yes, it is a complete and grammatical sentence.

Japanese often allows short sentences when the context is already known. English usually prefers more explicit information, but Japanese is very comfortable leaving things unstated if they are obvious.

So although it is short, 購入者が払いました。 is a perfectly valid sentence.