U toj trgovini prodaju svakakve stvari.

Because u meaning in a place takes the locative case.

So:

• base form: ta trgovina = that store
• after u for location: u toj trgovini = in that store

Both words change:

• ta → toj
• trgovina → trgovini

Croatian adjectives and demonstratives have to agree with the noun in gender, number, and case.

A useful comparison:

• u toj trgovini = in that store → location, so locative
• u tu trgovinu = into that store → movement, so accusative

Croatian usually uses u for being inside a building, room, store, town, country, and so on.

So:

• u trgovini = in the store
• u kući = in the house
• u gradu = in the city

Na is used for surfaces and also in some idiomatic place expressions:

• na stolu = on the table
• na poslu = at work
• na fakultetu = at the faculty / at university

A store is normally treated as an enclosed place, so u is the natural choice.

Croatian often leaves the subject unstated when it is clear from the verb form.

Prodaju is 3rd person plural, so the sentence literally has a kind of they sell structure.

But this they is often indefinite or generic, like in English:

• They sell good bread there.
• In that shop they sell all kinds of things.

It does not have to mean a specific group already mentioned. It can simply mean the people in that store, the staff, or just people there.

Prodaju is 3rd person plural present tense.

Basic idea:

• ja prodajem / prodam = I sell
• oni prodaju = they sell

In this sentence, it means they sell.

Because Croatian verb endings already show the person and number, you do not need a separate pronoun like oni unless you want emphasis.

In meaning, this sentence is understood as imperfective: a general, repeated, habitual action.

So here it is basically from prodavati = to sell, to be selling, to sell regularly.

That fits the context of describing what a store sells in general.

Why this can be confusing:

• prodavati and prodati can both produce the form prodaju in some contexts
• but context tells you which meaning is intended

Here, a store description like U toj trgovini prodaju svakakve stvari is naturally read as a habitual fact, so learners should understand it as imperfective.

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

The things are what is being sold, so they go in the accusative:

• prodaju što? → svakakve stvari

For this noun phrase, the accusative plural looks the same as the nominative plural:

• nominative plural: svakakve stvari
• accusative plural: svakakve stvari

That is very common with feminine plural forms.

Svakakve means something like:

• all kinds of
• all sorts of
• various
• all kinds of different

So:

• svakakve stvari = all sorts of things

Depending on tone and context, svakakve can sound:

• neutral: various kinds
• slightly loose or informal: this and that, all sorts of stuff

In this sentence, it most likely just means a wide variety of things.

Because the sentence is talking about many different items/things, not one thing.

• stvar = thing (singular)
• stvari = things (plural)

With svakakve, plural sounds very natural because the meaning is about variety:

• svakakve stvari = all sorts of things

A singular version would sound different and less natural here.

Yes. Croatian word order is fairly flexible.

The given sentence:

• U toj trgovini prodaju svakakve stvari.

could also be rearranged, for example:

• Prodaju svakakve stvari u toj trgovini.
• Svakakve stvari prodaju u toj trgovini.

The basic meaning stays similar, but the focus changes.

The original version starts with U toj trgovini, which puts the location first, as if saying:

• In that store, they sell all sorts of things.

So the word order is natural and probably chosen for emphasis on where this happens.

You can say a version with se, but it feels a bit different.

Compare:

• U toj trgovini prodaju svakakve stvari.
• U toj trgovini prodaju se svakakve stvari.

The version without se focuses more on unspecified people doing the selling:

• they sell all sorts of things there

The version with se is more impersonal and closer to:

• all sorts of things are sold there
• one sells all sorts of things there

Both are possible, but the sentence you were given is very natural when Croatian uses a general they.

In Croatian, the present tense can often cover both ideas, and context decides.

So prodaju can match English:

• they sell
• they are selling

In this sentence, because it sounds like a general description of what the store offers, the natural reading is:

• they sell
• they sell all kinds of things there

If you wanted to make it clearly about right now, Croatian would usually rely on context or add a time expression, such as sad = now.