Mi aĉetas la kolekton, malgraŭ tio, ke antikvaĵo havas makulon.

Because aĉeti means to buy, and the thing being bought is the direct object. In Esperanto, direct objects usually take -n.

• Mi aĉetas la kolekton = I am buying the collection.
• la kolekto without -n would not be marked as the object.

So the -n on kolekton is there because the collection is what the subject is buying.

La is the definite article, meaning the.

So:

• kolekto = collection
• la kolekto = the collection

In this sentence, the speaker is referring to a specific collection, not just any collection.

Esperanto has no indefinite article. There is a word for the (la), but there is no separate word for a or an.

So:

• antikvaĵo can mean an antique or just antique item, depending on context.
• la antikvaĵo would mean the antique.

That means antikvaĵo havas makulon can naturally mean an antique has a stain or the antique has a stain, depending on the situation and the intended meaning.

Antikvaĵo is built from:

• antikva = ancient, old, antique
• -aĵ- = a concrete thing made from or characterized by the root
• -o = noun ending

So antikvaĵo literally means something like an antique thing/object. In normal English, that is just an antique.

The suffix -aĵ- is very common in Esperanto for making a noun meaning a thing of that kind.

For the same reason as kolekton: it is the direct object.

In antikvaĵo havas makulon:

• antikvaĵo = the thing that has something
• makulon = the thing it has

Since the stain is what is being had, makulo becomes makulon.

• havi makulon = to have a stain

Because the sentence does not necessarily mean the stain. It just means a stain or stain in a general sense.

Compare:

• havas makulon = has a stain
• havas la makulon = has the stain, meaning a specific stain already known from context

Since Esperanto has no indefinite article, makulon by itself often corresponds to English a stain.

It means although, even though, or more literally despite the fact that.

Breakdown:

• malgraŭ = despite
• tio = that, that thing
• ke = that

So malgraŭ tio, ke... literally works like:

• despite that, that...
• more natural English: despite the fact that...
• simplest translation: although / even though...

This is a very common Esperanto structure for introducing a concessive clause.

Because malgraŭ normally takes a noun phrase, not a clause directly.
So Esperanto often uses tio as a kind of placeholder:

• malgraŭ tio = despite that
• ke antikvaĵo havas makulon = that an antique has a stain

Together:

• malgraŭ tio, ke antikvaĵo havas makulon = despite the fact that an antique has a stain

In practice, learners should treat malgraŭ tio, ke as a standard expression meaning although/even though.

Ke introduces a subordinate clause, usually translated as that.

Here it introduces the idea that follows tio:

• tio, ke antikvaĵo havas makulon = the fact that an antique has a stain

So ke connects the clause antikvaĵo havas makulon to the rest of the sentence.

In malgraŭ tio, ke..., the comma before ke is standard because ke introduces a subordinate clause.

The comma before malgraŭ separates the main clause from the concessive phrase:

• Mi aĉetas la kolekton = main clause
• malgraŭ tio, ke... = added idea: although/even though...

So the punctuation helps show the structure clearly.

Yes. Esperanto word order is fairly flexible, especially because case endings help show grammatical roles.

You could also say:

• Malgraŭ tio, ke antikvaĵo havas makulon, mi aĉetas la kolekton.

That means the same thing: Although an antique has a stain, I am buying the collection.

The main difference is emphasis. Starting with Malgraŭ tio, ke... puts the contrast first.

Yes. Kvankam also means although / even though.

For example:

• Mi aĉetas la kolekton, kvankam antikvaĵo havas makulon.

That is a simpler way to express the same basic idea.

Still, malgraŭ tio, ke is perfectly correct and has a slightly more literal sense of despite the fact that.

In Esperanto, the present tense -as can cover both simple present and present progressive, depending on context.

So:

• mi aĉetas can mean I buy
• or I am buying

Esperanto does not usually make a special grammatical distinction between those two the way English does.

It is grammatically understandable, but a native speaker might wonder exactly what is meant by antikvaĵo here.

Without more context, antikvaĵo havas makulon sounds like an antique has a stain. If you mean one specific antique already known, la antikvaĵo might be more natural. If you mean one item within the collection, something like unu antikvaĵo might also be clearer.

For example:

• Mi aĉetas la kolekton, malgraŭ tio, ke unu antikvaĵo havas makulon.
  = I am buying the collection, even though one antique has a stain.

So the original sentence is workable, but context would determine whether a small adjustment would sound more natural.