Prima di comprare un libro, confronto il prezzo in libreria con quello online.

Because in Italian, prima di + infinitive means before doing something.

So:

• prima di comprare = before buying
• prima di uscire = before going out
• prima di parlare = before speaking

You normally need di before the infinitive here.
So prima comprare would sound incomplete or wrong in this meaning.

After prima di, Italian usually uses the infinitive when the subject is the same as in the main clause.

In this sentence:

• Prima di comprare un libro = Before buying a book
• confronto il prezzo... = I compare the price...

The person who buys and the person who compares is the same person: I.

If the subject changes, Italian often uses prima che + subjunctive instead:

• Prima che tu compri un libro, confronto i prezzi would not fit this exact meaning naturally, but it shows the different structure.
• More generally: Prima che lui parta, voglio parlargli = Before he leaves, I want to speak to him

Confronto is the 1st person singular present indicative of confrontare.

So:

• io confronto = I compare
• tu confronti = you compare
• lui/lei confronta = he/she compares

In Italian, the subject pronoun io is often omitted because the verb ending already shows the subject.
So confronto by itself already means I compare.

Italian often drops subject pronouns when they are not needed.

So instead of saying:

• Io confronto il prezzo...

it is very natural to say:

• Confronto il prezzo...

The ending -o tells you it is I.
You would add io only for emphasis, contrast, or clarity.

For example:

• Io confronto il prezzo, tu no. = I compare the price, you don't.

Because they play different roles in the sentence.

• un libro means a book in a general sense: any book, not a specific one.
• il prezzo means the price, because it refers to the specific price of that book in the situation being discussed.

Italian often uses the definite article where English also does, but sometimes more regularly than English.

Here the idea is:

• before buying a book, I compare the price...

That sounds natural because once the book is introduced, the price being compared is understood as the price of that book.

Yes, libreria can mean bookcase/bookshelf, but it also commonly means bookstore.

So:

• in libreria = in/at the bookstore
• una libreria in salotto = a bookcase in the living room

Context tells you which meaning is intended.

In this sentence, since we are talking about comparing prices before buying a book, in libreria clearly means at the bookstore.

Because in libreria is the normal idiomatic way to say in/at the bookstore.

Italian often uses in with certain places:

• in libreria
• in banca
• in ufficio
• in farmacia

Alla libreria is generally not the usual choice here.

Also, in libreria can sometimes mean either:

• physically inside the bookstore
• more generally, in the bookstore context/as sold in bookstores

Here quello stands for il prezzo so that Italian does not have to repeat the noun.

The full idea is:

• confronto il prezzo in libreria con il prezzo online

But repeating il prezzo sounds heavier. Italian often replaces the repeated noun with a demonstrative pronoun:

• quello = that one / the one

So:

• con quello online = with the online one
  meaning with the online price

Because quello refers back to il prezzo, which is:

• masculine
• singular

So the pronoun must match it:

• il prezzo → quello

If the noun were feminine singular, you would use quella.
If plural, quelli or quelle, depending on gender.

Examples:

• la tariffa ... quella online
• i prezzi ... quelli online
• le offerte ... quelle online

Because online is describing the implied noun: the online one.

So:

• quello online = the one online

This is a very natural way in Italian to avoid repeating the noun.

You could think of it as short for:

• quello disponibile online
• quello su internet

But quello online is concise and idiomatic.

Because confrontare commonly takes con when you compare one thing with another:

• confrontare X con Y = to compare X with Y

So here:

• confronto il prezzo in libreria con quello online

means:

• I compare the bookstore price with the online one

This is the standard pattern.

In standard Italian, confronto from confrontare is the safer and more natural choice here.

For to compare, the most common verbs are:

• confrontare
• paragonare (often more like to liken / compare in a broader sense)

The form comparare exists, but it is less common in everyday speech and can sound more technical or less idiomatic depending on context.

So for a learner, confrontare is the best option here.

It is not always strictly necessary, but it is very normal and helpful.

The introductory phrase:

• Prima di comprare un libro

sets the scene before the main clause:

• confronto il prezzo in libreria con quello online

The comma makes the sentence easier to read. In short sentences, Italian punctuation can sometimes be flexible, but this comma is perfectly natural.

Yes. Both are possible.

For example:

• confronto il prezzo in libreria con quello su internet
• confronto il prezzo in libreria con quello online

Both mean essentially the same thing.
Online is very common in modern Italian and sounds natural here.

The structure is:

• Prima di comprare un libro = introductory time phrase
• confronto = main verb
• il prezzo in libreria = direct object
• con quello online = comparison phrase

So literally it works like:

• Before buying a book, I compare the bookstore price with the online one.

This is a very typical Italian pattern: an introductory phrase followed by the main clause.