Wir merken, dass das Schnäppchen sichtbare Gebrauchsspuren hat.

Because dass introduces a subordinate clause. In German, subordinate clauses are normally separated from the main clause with a comma.

• Main clause: Wir merken
• Subordinate clause: dass das Schnäppchen sichtbare Gebrauchsspuren hat

So the comma is required.

Because after dass, German uses subordinate clause word order, and the conjugated verb usually goes to the end of the clause.

Compare:

• Main clause: Das Schnäppchen hat sichtbare Gebrauchsspuren.
• Subordinate clause: ..., dass das Schnäppchen sichtbare Gebrauchsspuren hat.

This is one of the most important word order patterns in German.

They are different words:

• dass = that as a conjunction
• das = the/that as an article or pronoun

In this sentence:

• dass introduces the subordinate clause
• das Schnäppchen uses das as the neuter definite article

A useful test: if you can replace it with welches or dieses, it is often das. If it simply links two clauses, it is dass.

Because Schnäppchen is a neuter noun in German, so its article is das.

• singular: das Schnäppchen
• plural: die Schnäppchen

German noun gender has to be learned with the noun, so it is best to memorize das Schnäppchen as a full unit.

Schnäppchen means a bargain or great deal. It can refer to something bought cheaply or something considered especially good value.

In this sentence, it refers to the item that turned out to have signs of wear. So the idea is something like: the bargain item has visible wear marks.

Because it is an adjective before a noun, and German adjectives change their endings depending on:

• gender
• number
• case
• the article before them

Here we have:

• das Schnäppchen = neuter singular nominative subject
• sichtbare Gebrauchsspuren = plural accusative object with no article

Because Gebrauchsspuren is plural and there is no article before it, the adjective takes the strong ending -e in the plural accusative: sichtbare Gebrauchsspuren.

Because Gebrauchsspuren means signs of wear, wear marks, or traces of use, and this idea is very often expressed in the plural in German.

• singular: die Gebrauchsspur
• plural: die Gebrauchsspuren

So sichtbare Gebrauchsspuren means visible signs of wear.

Because the verb agrees with the subject, not with the object.

The subject is:

• das Schnäppchen = singular

The object is:

• sichtbare Gebrauchsspuren = plural

So the verb must be singular: hat.

Because merken and sich merken mean different things.

• merken = to notice, realize
• sich merken = to remember, memorize

So here:

• Wir merken = We notice / We realize

If you said Wir merken uns ..., that would suggest we remember ..., which is a different meaning.

Yes, in many contexts you could say Wir bemerken, dass ... and the meaning would be very similar.

A rough difference:

• merken often sounds a bit more natural and everyday
• bemerken can sound slightly more formal or more explicitly like to notice

So this sentence with merken is completely normal and idiomatic.

It is the accusative plural, because it is the direct object of hat.

You can see the structure like this:

• das Schnäppchen = subject, nominative
• hat = verb
• sichtbare Gebrauchsspuren = direct object, accusative

Since plural nouns often look the same in nominative and accusative, the case is understood from the sentence structure.

Yes. It is a very common pattern in German:

• Wir merken, dass ...
• Ich merke, dass ...
• Man merkt, dass ...

It is used when someone becomes aware of something. Very common alternatives are:

• Wir sehen, dass ... = we see that ...
• Wir stellen fest, dass ... = we determine/find that ...
• Wir bemerken, dass ... = we notice that ...

Yes. The subordinate clause can be turned into an independent sentence, but then the word order changes back to normal main clause order.

Subordinate clause:

• ..., dass das Schnäppchen sichtbare Gebrauchsspuren hat.

Independent sentence:

• Das Schnäppchen hat sichtbare Gebrauchsspuren.

So the meaning stays basically the same, but the verb moves from the end back to the second position.

Because Gebrauchsspuren haben is a very natural German expression for an object showing wear from use.

• Gebrauchsspuren haben = to have signs of wear
• sichtbare Gebrauchsspuren haben = to have visible signs of wear

This is more idiomatic for physical condition than trying to translate English word-for-word. German often prefers a noun phrase here rather than an adjective-based structure.