Se a ottobre il mercato vende ancora albicocche e ciliegie, ne comprerò un po’ per la torta di domenica.

Because Italian normally uses se + present indicative for a real or possible future condition.

So:

• Se ... vende, comprerò ...

is the standard pattern for If ... sells / is selling ..., I will buy ...

Using the future after se here, such as se venderà, is generally not the normal standard choice in this kind of sentence.

Yes, both a ottobre and in ottobre can mean in October.

In this sentence, a ottobre is a very natural way to place the action in that month. Italian often uses a with months and seasons in time expressions.

Also, notice there is no article before ottobre. With months, Italian usually does not use an article when simply saying in October, in May, and so on.

In Italian, just like in English, a place or institution can stand for the people working there.

So il mercato vende means something like:

• the market is selling
• the market still has for sale
• the vendors at the market are selling

It is a normal collective use of il mercato, even though the actual sellers are the people at the market.

Because they are indefinite plural objects.

Here the sentence is not talking about specific, already identified apricots and cherries. It just means the market is still selling those fruits in general.

So:

• vende ancora albicocche e ciliegie = still sells apricots and cherries

Italian often allows this kind of bare plural, especially after verbs like vendere, comprare, mangiare, and similar verbs.

Here it means still.

So the idea is:

• If in October the market still sells apricots and cherries...

That makes sense because those fruits may no longer be in season by then.

In other contexts, ancora can also mean again, but here still is clearly the right meaning.

Ne is a pronoun that often means of it, of them, or some of it/them, depending on context.

Here it refers back to albicocche e ciliegie.

So:

• ne comprerò un po’ = I’ll buy some of them

It avoids repeating the nouns:

• comprerò un po’ di albicocche e ciliegie
• ne comprerò un po’

Both express the same basic idea, but the second is more elegant because it avoids repetition.

Yes.

Ne can refer to the whole previously mentioned set:

• albicocche e ciliegie

So ne comprerò un po’ means I’ll buy some of those fruits. It does not have to mean only one of the two. It can refer to both together.

Because object pronouns in Italian normally come before a conjugated verb.

So you get:

• ne comprerò
• lo vedo
• la faccio
• li porto

This is the regular word order with pronouns in everyday Italian.

Un po’ means a little, a bit, or some.

In this sentence:

• ne comprerò un po’ = I’ll buy some

The apostrophe is there because po’ is a shortened form of poco. It is not an accent mark. The standard spelling is po’, not pò.

Because it describes the result or consequence of the condition.

The sentence structure is:

• If X happens, I will do Y

So:

• Se ... vende ..., ne comprerò ...

That future tense makes the speaker’s intention clear: if the fruit is available in October, then the speaker will buy some.

Per here means for, expressing purpose.

So the fruit will be bought for the cake.

Then di domenica describes the cake:

• la torta di domenica = the Sunday cake / the cake for Sunday

A natural English translation is for Sunday’s cake or for the cake on Sunday, depending on context.

Because here it is attached to the noun torta.

Compare:

• Di domenica riposo.
  = On Sundays I rest.
  Here it is an adverbial time expression.

• La torta di domenica
  = the Sunday cake / the cake for Sunday
  Here it describes the noun torta.

So in this sentence, di domenica is not talking about a habitual action every Sunday. It is identifying which cake the speaker means.