Što se rasprodaje tiče, blagajnica kaže da traje samo do subote.

It means as for the sale, regarding the sale, or when it comes to the sale.

This is a very common Croatian expression for introducing a topic. You can think of što se X tiče as a fixed pattern:

• Što se vremena tiče = as for the weather
• Što se cijene tiče = as for the price

So in your sentence, it sets up the topic before the main statement: As for the sale, the cashier says it lasts only until Saturday.

Because this expression requires the genitive case.

The noun is:

• rasprodaja = sale, clearance sale

But in što se ... tiče, the noun goes into the genitive:

• rasprodaja → rasprodaje

So:

• Što se rasprodaje tiče = as for the sale

This is because the verb behind the expression is ticati se, which takes the genitive.

Se is part of the verb expression ticati se, which means to concern or to be about.

So the full underlying idea is something like:

• To se tiče rasprodaje = that concerns the sale

In the fixed phrase što se X tiče, you do not usually translate se separately. It is just part of the structure. For learners, it is best to memorize što se ... tiče as one chunk.

Blagajnica usually means cashier or female checkout clerk.

In a shop context, it is the woman working at the register. Depending on context, English might translate it as:

• cashier
• checkout clerk
• store clerk

The masculine form is blagajnik.

Because da introduces a subordinate clause, similar to English that.

So:

• blagajnica kaže = the cashier says
• da traje samo do subote = that it lasts only until Saturday

Croatian often uses da where English uses that, although in English that is often omitted.

So literally:

• The cashier says that it lasts only until Saturday.

Traje comes from the verb trajati, which means to last, to continue, or to go on.

Here it means:

• traje = lasts / is on

It is in the 3rd person singular present tense because the implied subject is rasprodaja:

• rasprodaja traje = the sale lasts

Even though rasprodaja is not repeated in the second clause, it is understood from the context.

Because Croatian often leaves out words that are already clear from context.

In full, you could say:

• Blagajnica kaže da rasprodaja traje samo do subote.

But since rasprodaja has already been mentioned, Croatian naturally omits it:

• Blagajnica kaže da traje samo do subote.

English does something similar with it: The cashier says it lasts only until Saturday.

Croatian can simply leave the subject implied.

Do subote means until Saturday.

The preposition do takes the genitive case, so:

• subota = Saturday
• subote = of Saturday / until Saturday after do

So:

• do subote = until Saturday

Also note that in Croatian, days of the week are normally written with a lowercase letter, so subote, not Subote, unless it begins a sentence.

Yes, it is normal and expected here.

Što se rasprodaje tiče is an introductory topic phrase, so it is separated from the main clause by a comma:

• Što se rasprodaje tiče, blagajnica kaže...

This works much like English:

• As for the sale, the cashier says...

The comma helps show that the first part is setting the topic, not forming the main statement yet.

Yes. A more direct version would be:

• Blagajnica kaže da rasprodaja traje samo do subote.

This means the same thing:
The cashier says the sale lasts only until Saturday.

The original sentence uses Što se rasprodaje tiče to emphasize the topic, as if the speaker is specifically addressing the question of the sale. So the original feels a bit more like:

• As for the sale, the cashier says it lasts only until Saturday.

Both are correct, but the original has more of a topic-introducing feel.