Коллекционеры обменялись антиквариатом.

Коллекционеры is the nominative plural form of коллекционер.

It is in the nominative because it is the subject of the sentence, and it is plural because there is more than one collector.

• singular: коллекционер
• plural: коллекционеры

So Коллекционеры = the collectors.

Обменялись is the past tense, plural form of обменяться.

In this sentence, it means exchanged with each other or swapped.

A useful breakdown:

• обменяться = to exchange, to swap
• обменялись = they exchanged / they swapped

Because the subject is plural, the verb naturally suggests a mutual action: the collectors exchanged things among themselves.

The ending -сь is a variant of -ся. It is added to many Russian verbs.

Here it does not mean a literal reflexive action like washed themselves. Instead, with обменяться, it gives the idea of a reciprocal action:

• обменяться = to exchange with one another

So обменялись means not just exchanged, but exchanged with each other.

Also, -сь is used instead of -ся after a vowel:

• обменялись not
• обменялися

Because the verb обменяться normally takes the thing exchanged in the instrumental case.

The pattern is:

• обменяться чем? = to exchange what?

So:

• антиквариат = antiques / antique goods
• антиквариатом = with antiques / in antiques-as-the-thing-exchanged

This is just the standard grammar of this verb.

Compare:

• обменяться книгами = to exchange books
• обменяться подарками = to exchange gifts
• обменяться антиквариатом = to exchange antiques

Because обменяться does not take a direct object in the accusative the way many English verbs do.

In English, we say exchange antiques, but in Russian the construction is different:

• обменяться чем?
  • instrumental

So Russian is not treating антиквариат here as a direct object. It is treating it as the thing used in the exchange, which is why the instrumental is required.

Grammatically, антиквариат is singular, and антиквариатом is its instrumental singular form.

But in meaning, it often works like a collective or mass noun, similar to English antiques or antique goods. So it does not have to mean just one antique item.

In other words, this sentence can easily mean that the collectors exchanged antique items in general, not one single object.

This is a question of aspect.

• обменялись = perfective
• обменивались = imperfective

Here, обменялись presents the exchange as a completed event.

So:

• Коллекционеры обменялись антиквариатом. = The collectors exchanged antiques. The action is seen as finished.
• Коллекционеры обменивались антиквариатом. = The collectors were exchanging antiques / used to exchange antiques. This focuses on process, repetition, or background action.

Not with the same meaning.

Обменяли comes from обменять, which is a different construction. It is usually used when you say that someone exchanged something for something else:

• обменять антиквариат на картины = to exchange antiques for paintings

So:

• обменялись антиквариатом = they exchanged antiques with each other
• обменяли антиквариат = they exchanged/traded away the antiques, often with the idea that something else was received in return

The version with обменяли often feels incomplete unless you add на что.

Not separately, because that idea is already built into обменялись.

With a plural subject, обменялись normally implies that the people involved exchanged something with one another.

If you want to name an outside exchange partner, you can add с + instrumental:

• Коллекционеры обменялись антиквариатом с музеем.

But without that, the natural reading is that the collectors exchanged antiques among themselves.

The word order here is fairly neutral:

• Коллекционеры обменялись антиквариатом.

Because Russian uses cases, the word order is more flexible than in English. You can move things around for emphasis.

For example:

• Антиквариатом коллекционеры обменялись.
  This emphasizes what they exchanged.

The original version is the most straightforward, neutral way to say it.

The normal stress is:

• Коллекционе́ры обменя́лись антиквариа́том.

A few pronunciation notes:

• Коллекционе́ры has stress on е
• обменя́лись has stress on я
• антиквариа́том has stress on а

The final -сь in обменялись is a soft sound, roughly like a soft s.

Because Russian has no articles.

So Russian does not have separate words matching English the and a/an. Whether something is definite or indefinite is usually understood from:

• context
• word order
• emphasis
• general meaning

So Коллекционеры обменялись антиквариатом can mean something like:

• The collectors exchanged antiques
• Collectors exchanged antiques

The exact English choice depends on context, not on a separate Russian article.