إعلان البيع في الرسالة.

Usually, no. By itself, it is most naturally a noun phrase, not a full sentence.

It works well as:

• a title
• a label
• a caption
• a short heading

So Arabic can use a phrase like this where English might also use a short heading such as Sale announcement in the message.

إعلان البيع is an iḍāfa construction, often explained as a noun + noun structure meaning X of Y.

Here:

• إعلان = announcement
• البيع = the sale

So literally, it is announcement of the sale.

In more natural English, that may become:

• the sale announcement
• the announcement of the sale

In an iḍāfa, the first noun is the main thing, and the second noun tells you what it is connected to.

Because in an iḍāfa, the first noun normally does not take ال.

So Arabic says:

• إعلان البيع

not:

• الإعلان البيع

That second version is not the normal structure for this meaning.

Even without ال on إعلان, the whole phrase can still be definite because definiteness in an iḍāfa is determined by the second noun.

Because البيع is the second part of the iḍāfa, and it is definite: the sale.

That makes the whole phrase definite:

• إعلان البيع = the announcement of the sale

Compare:

• إعلان البيع = the sale announcement / the announcement of the sale
• إعلان بيع = a sale announcement / an announcement of a sale

So adding ال to the second noun changes how specific the whole phrase sounds.

في الرسالة is a prepositional phrase meaning in the message or in the letter, depending on context.

It modifies the whole noun phrase:

• إعلان البيع = the sale announcement
• في الرسالة = in the message

So the full phrase means something like:

• the sale announcement in the message

The preposition في means in, and the noun after it is in the genitive case.

Because ر is a sun letter.

When ال comes before a sun letter, the l sound of ال is not pronounced separately. Instead, the following consonant is emphasized or doubled.

So:

• الرسالة is pronounced ar-risālah
• not fully pronounced as al-risālah

This is a pronunciation rule only. In writing, it still stays الرسالة.

If you fully vowel the phrase, one possible form is:

إعلانُ البيعِ في الرسالةِ

A rough pronunciation is: iʿlānu l-bayʿi fī r-risālati

Why these endings?

• البيعِ is genitive because it is the second term of an iḍāfa
• الرسالةِ is genitive because it comes after في
• إعلانُ would be nominative if the phrase is being cited by itself or functioning as a subject

Important: in normal written Arabic, these short vowels are usually not written, so learners often see just:

إعلان البيع في الرسالة

إعلان is a verbal noun or maṣdar from the verb أعلن, which means to announce or to declare.

So:

• أعلن = he announced / to announce
• إعلان = announcement / announcing

This is very common in Arabic: a verbal noun often functions like an ordinary noun in phrases such as this one.

Not always. رسالة can mean:

• message
• letter
• sometimes even treatise or epistle in formal contexts

So في الرسالة could mean:

• in the message
• in the letter

The exact choice depends on context. In Modern Standard Arabic, both message and letter are very possible meanings.