Vi skal snart bestemme, hvad der skal sælges, og hvad det må koste på loppemarkedet.

Because Danish main clauses normally follow the V2 rule: the finite verb comes in the second position.

So the order is:

• Vi = subject
• skal = finite verb
• snart = adverb
• bestemme = infinitive

That is why Vi skal snart bestemme is the natural order.

A form like Vi snart skal bestemme is not the normal neutral word order in Danish.

Both instances of skal are forms of the modal verb meaning something like shall, have to, or be going to, but the exact English wording changes with context.

• Vi skal snart bestemme = We need to / have to / are going to decide soon
• hvad der skal sælges = what is to be sold / what should be sold

So it is the same Danish modal verb, but English translates it differently depending on the clause.

Because this is an embedded question after bestemme.

Compare:

• Direct question: Hvad skal sælges?
• Embedded question: Vi skal bestemme, hvad der skal sælges.

In Danish, embedded questions do not keep the same word order as direct questions.

Also, der appears here because hvad functions as the subject of the passive clause. This is similar to:

• Jeg ved, hvem der kommer = I know who is coming

So hvad der skal sælges is the normal Danish structure.

Here der is not the word meaning there.

It is a grammatical marker often used when a question word or relative word is the subject of the clause. In this sentence, the thing being sold is what the clause is about, so Danish uses der.

A useful comparison:

• Hvem kommer? = Who is coming?
• Jeg ved, hvem der kommer = I know who is coming

So in hvad der skal sælges, the der helps introduce the subject role inside the embedded clause.

Because det is the subject of koste.

Literally, the structure is what it may cost:

• hvad = what / how much
• det = it
• må koste = may cost / is allowed to cost

So hvad is asking about the price, while det refers to the item.

This is different from hvad der skal sælges, where hvad itself is tied to the subject role in the passive clause.

Here må means may in the sense of is allowed to.

So hvad det må koste means something like:

• what it may cost
• what it is allowed to cost
• how much they are allowed to charge for it

It does not mean must here. Danish må can sometimes mean must, but this context is about deciding prices, so the meaning is clearly about permission or an allowed limit.

Yes, you could say hvad vi skal sælge.

The difference is mainly one of focus:

• hvad vi skal sælge = focuses on we as the people doing the selling
• hvad der skal sælges = focuses on the things that are to be sold

sælges is an -s passive form:

• sælge = to sell
• sælges = be sold

This passive is very common in Danish, especially in neutral or slightly formal language.

With koste, Danish very often uses hvad to ask about price.

For example:

• Hvad koster det? = How much does it cost?

So hvad det må koste is completely natural.

You may also hear hvor meget, and it is understandable, but hvad is very idiomatic with koste in Danish.

Because Danish usually says på with things like markets, fairs, and similar events or venues.

So:

• på loppemarkedet = at the flea market

Using i would sound less idiomatic here, because the idea is not mainly being physically inside something, but being at the market as an event or place.

Danish forms compound nouns as one word.

So:

• loppe = flea
• marked = market
• loppemarked = flea market

Then -et is the definite ending for a neuter noun:

• et loppemarked = a flea market
• loppemarkedet = the flea market

This is very typical Danish noun building.

The commas help separate the main clause from the two embedded question clauses:

• Vi skal snart bestemme
• hvad der skal sælges
• og hvad det må koste på loppemarkedet

Danish often marks clause boundaries with commas more clearly than English does. In this sentence, the commas show that what is to be sold and what it may cost are both things that must be decided.