syuppinsya ga messeezi wo kakimasu.

A common reading is:

• 出品者 = しゅっぴんしゃ (shuppinsha)
• が = ga
• メッセージ = messēji
• を = o (even though it is written with the kana を)
• 書きます = かきます (kakimasu)

So the full sentence is:

しゅっぴんしゃ が メッセージ を かきます。

出品者 means the person who listed an item for sale, so in many contexts it is best translated as seller or listing seller.

It is built from:

• 出品 = listing an item / putting an item up for sale
• 者 = person

So 出品者 literally means person who listed the item.

This is especially common on online marketplace sites and auction platforms.

が marks the subject of the sentence.

In this sentence:

• 出品者が = the seller is the one doing the action
• メッセージを = the message is the thing affected by the action
• 書きます = writes / will write

So が tells you who performs the action.

For a beginner, a useful way to think about it is:

• が = marks who/what does it
• を = marks what is acted on

That is a very common question.

Both can be possible, but they do different jobs:

• が marks the grammatical subject
• は marks the topic

So:

• 出品者がメッセージを書きます = The seller writes the message / It is the seller who writes the message
• 出品者はメッセージを書きます = As for the seller, they write a message

Using が can sound more like you are identifying who does the action, especially if that matters in context.

Using は would make 出品者 the topic of discussion rather than simply the subject.

At the beginner level, it is enough to remember:

• が = subject marker
• は = topic marker

を marks the direct object of the verb.

Here:

• メッセージを = the message is the thing being written

So the structure is:

• 出品者が = the seller (subject)
• メッセージを = the message (object)
• 書きます = writes

This is very common with action verbs in Japanese.

Examples:

• 本を読みます = read a book
• 手紙を書きます = write a letter
• 水を飲みます = drink water

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

So while English often looks like:

• Subject + Verb + Object
• The seller writes a message

Japanese often looks like:

• Subject + Object + Verb
• 出品者が メッセージを 書きます

This is one of the biggest word-order differences between English and Japanese.

A very useful beginner pattern is:

A が B を Verb

meaning:

A does Verb to B

書きます is the polite present/future form of the verb 書く (to write).

• 書く = plain/dictionary form
• 書きます = polite form

Japanese often uses 〜ます in normal polite speech, textbooks, customer service situations, and many neutral everyday situations.

So:

• 出品者がメッセージを書く。 = plain style
• 出品者がメッセージを書きます。 = polite style

The meaning is basically the same, but the tone is different.

It can mean writes or will write, depending on context.

Japanese non-past forms often cover both:

• present habitual
• near future

So 書きます could mean:

• writes
• is going to write
• will write

You usually figure it out from the situation.

For example:

• If describing a routine, it may mean writes
• If talking about what happens next, it may mean will write

Japanese often leaves that kind of time information to context unless a time word is added.

Yes. メッセージ is a loanword written in katakana, the script often used for foreign-derived words.

It comes from English message.

In modern Japanese, メッセージ is very common, especially in digital contexts like:

• apps
• email
• online marketplaces
• chat systems

So even though it looks foreign, it is a normal Japanese vocabulary item.

You are right: normal Japanese is usually written without spaces.

So the natural written form is:

出品者がメッセージを書きます。

Spaces are often added in beginner materials to make the sentence easier to read and to help learners see the particles:

• 出品者
• が
• メッセージ
• を
• 書きます

So the spaces are a teaching aid, not standard Japanese writing.

By itself, 出品者 does not clearly mark singular vs. plural.

Depending on context, it could mean:

• the seller
• a seller
• sometimes even sellers

Japanese nouns usually do not change form for singular/plural the way English nouns do.

If Japanese wants to make plurality clear, it often does so through context or extra words.

So in this sentence, English may translate it as the seller or a seller, depending on the situation.

Japanese does not have articles like English a, an, and the.

That means 出品者 can mean:

• a seller
• the seller

And メッセージ can mean:

• a message
• the message

Context tells you which English article sounds best in translation.

This is very normal in Japanese, and learners gradually get used to supplying a/the only when translating into English.

Sometimes, yes, especially in casual conversation, but not always.

For example, people may say things like:

• 出品者、メッセージ書きます
• メッセージ書きます

if the meaning is obvious from context.

However, particles are important for clarity, and in textbook Japanese or careful speech, using them is best.

For learners, it is a good idea to keep using the particles correctly until you become comfortable with when omission sounds natural.

Yes, but it changes the verb and therefore the nuance.

• 書きます = writes
• 送ります = sends

So:

• 出品者がメッセージを書きます = The seller writes a message.
• 出品者がメッセージを送ります = The seller sends a message.

In many real situations, someone may write and then send a message, but they are not exactly the same action.

So the original sentence focuses on writing/composing the message.

Yes, it is grammatically natural.

That said, whether it sounds completely natural in real life depends on context.

For example, in an online marketplace context, people often also say things like:

• 出品者がメッセージを送ります = The seller sends a message.
• 出品者からメッセージが来ます = A message comes from the seller.

But 出品者がメッセージを書きます is still a normal, understandable sentence and works well for learning basic grammar:

• subject marked by が
• object marked by を
• verb at the end
• polite verb form 〜ます