Ispravna stvar ima trag korišćenja.

As written, ispravna stvar is one phrase: a/the functional item/thing.

So the sentence is structured like this:

• ispravna stvar = the functional item
• ima = has
• trag korišćenja = a sign/trace of use

If you want to say The item is functional, Serbian would normally use je:

• Stvar je ispravna.
• More naturally in a listing: Predmet je ispravan.

Because it agrees with stvar, which is a feminine singular noun.

In Serbian, adjectives change to match the noun’s:

• gender
• number
• case

Here:

• ispravna stvar = feminine singular
• ispravan predmet = masculine singular
• ispravno pakovanje = neuter singular

So the -a ending is just normal adjective agreement.

It can mean either, depending on context. In everyday Serbian, ispravan can mean:

• correct / proper
• working / functional / not broken

In a sentence about an item and signs of use, the intended meaning is usually working or in proper condition, not morally right or correct.

Because Serbian does not have articles.

So stvar can mean:

• a thing
• the thing
• sometimes just thing/item

The exact meaning depends on context. English requires articles, but Serbian usually does not.

Korišćenja is genitive singular of korišćenje.

Base form:

• korišćenje = use, usage

In the phrase trag korišćenja, the second noun goes into the genitive because it means trace/sign of use.

So:

• trag = trace, sign
• korišćenja = of use

This is a very common Serbian noun + genitive pattern.

It is understandable, but many speakers would more naturally say tragove korišćenja or znakove korišćenja in this kind of context.

Very common listing-style phrases are:

• ima tragove korišćenja = it has signs of use
• vide se tragovi korišćenja = signs of use are visible
• sa tragovima korišćenja = with signs of use

The singular trag sounds more like one trace/mark. If you mean general wear, the plural is more idiomatic.

It is understandable, but it is very generic. Stvar literally means thing.

In product descriptions, Serbian more often uses words like:

• predmet = item
• artikl = article/item
• a specific noun such as telefon, stolica, uređaj, etc.

So while ispravna stvar is grammatical, something like predmet je ispravan i ima tragove korišćenja would sound more natural in many real situations.

Yes. A more natural version in many contexts would be:

• Stvar je ispravna i ima tragove korišćenja.
• Predmet je ispravan i ima tragove korišćenja.

This is because Serbian often separates the two ideas:

1. the item is working
2. it shows signs of use

Your original sentence is understandable, but it sounds a bit compressed and slightly less natural than these alternatives.

Yes. Tragovi upotrebe is also very common, and many speakers may even prefer it in product descriptions.

Both work:

• tragovi korišćenja
• tragovi upotrebe

They are very close in meaning. In this kind of sales or condition-related language, upotreba often sounds a little more standard or fixed, but korišćenje is perfectly understandable too.