Na tej wyprzedaży każda przecena wygląda dobrze, ale nie wszystko jest naprawdę tanie.

In Polish, na wyprzedaży is the normal idiomatic way to say at a sale / in a sale context.

• na
  • locative often appears with events, activities, and commercial contexts:
    • na koncercie = at the concert
    • na promocji = on promotion / in a sale
    • na wyprzedaży = at the sale

So na tej wyprzedaży means at this sale / in this sale.

The form tej is locative singular feminine, because wyprzedaż is a feminine noun, and na here requires the locative.

They are related, but not the same.

• wyprzedaż = a sale, especially a bigger sales event or clearance sale
• przecena = a price reduction, markdown, or discounted item/discount

So in this sentence:

• Na tej wyprzedaży = At this sale
• każda przecena = every markdown / every discount

A natural way to understand it is that the whole event is a wyprzedaż, and within it there are individual przeceny.

Because przecena is a feminine noun.

The word każdy changes to match the gender and case of the noun:

• masculine: każdy
• feminine: każda
• neuter: każde

Since przecena is feminine singular, we use każda przecena.

Polish often uses singular after words like każdy / każda / każde (every / each), just like English does:

• every discount
• not every discounts

So:

• każda przecena wygląda dobrze = every discount looks good

Even though the meaning refers to many discounts, grammatically the phrase is singular.

Because the subject is każda przecena, which is grammatically singular.

• każda przecena = every discount
• therefore: wygląda = looks (3rd person singular)

If the subject were plural, the verb would also be plural, for example:

• wszystkie przeceny wyglądają dobrze = all the discounts look good

Wszystko means everything, and it is grammatically singular and neuter. That is why the verb is jest and the adjective is tanie.

So:

• wszystko = everything
• nie wszystko = not everything
• jest = is
• tanie = cheap (neuter singular form)

By contrast, wszystkie would need a plural noun:

• nie wszystkie produkty są naprawdę tanie = not all products are really cheap

In your sentence, nie wszystko is more general and leaves the noun unstated.

Because tanie agrees with wszystko, which is grammatically neuter singular.

Adjectives in Polish must agree with the noun or pronoun they describe:

• masculine singular: tani
• feminine singular: tania
• neuter singular: tanie
• plural: tanie / tani depending on the group

Since wszystko is treated as neuter singular, we say:

• wszystko jest tanie = everything is cheap

Yes, literally it means looks good, and here it suggests seems attractive, appears to be a good deal, or looks appealing.

So każda przecena wygląda dobrze does not necessarily mean the discounts truly are good. It means they appear good at first glance.

That contrast is important because the second part says:

• ale nie wszystko jest naprawdę tanie = but not everything is really cheap

So the sentence contrasts appearance with reality.

Naprawdę means really / truly / actually.

Here it strengthens the contrast:

• something may look cheap because it has been marked down
• but it may not be really cheap in reality

So nie wszystko jest naprawdę tanie means something like:

• not everything is genuinely cheap
• not everything is actually cheap

Yes, you could, but the emphasis changes slightly.

• każda przecena wygląda dobrze = every single discount looks good
• wszystkie przeceny wyglądają dobrze = all the discounts look good

Both are natural.
Każda przecena sounds a bit more individual and emphatic, focusing on each markdown one by one.
Wszystkie przeceny sounds a bit more collective.

Yes. Ale is the standard everyday word for but.

The sentence has a very natural contrast:

• każda przecena wygląda dobrze = every markdown looks good
• ale = but
• nie wszystko jest naprawdę tanie = not everything is really cheap

So ale works exactly as you would expect in English.

Yes, this word order is natural and idiomatic.

Polish word order is more flexible than English, but the original order sounds smooth and neutral:

• Na tej wyprzedaży każda przecena wygląda dobrze, ale nie wszystko jest naprawdę tanie.

You could change it for emphasis, for example:

• Każda przecena na tej wyprzedaży wygląda dobrze...
• Ale nie wszystko na tej wyprzedaży jest naprawdę tanie.

These are also possible, but they shift the focus slightly. The original version is a very normal, balanced sentence.