Við semjum um verð við söluaðilann.

They are two different words that just happen to be spelled the same.

• Við at the start = we (subject pronoun)
• við before söluaðilann = with (preposition)

The first one is capitalized only because it starts the sentence. Icelandic often relies on context to tell these apart.

Semjum is the 1st person plural present tense form of semja.

So:

• semja = to negotiate, arrange, compose, make an agreement, depending on context
• við semjum = we negotiate

In this sentence, because of the full pattern semja við einhvern um eitthvað, it clearly means negotiate.

Because semja often works with this pattern:

semja við einhvern um eitthvað

That means:

• við einhvern = with someone
• um eitthvað = about something / over something

So in your sentence:

• við söluaðilann = with the seller/dealer
• um verð = about the price

A very literal breakdown is:

We negotiate with the seller about price.

Because Icelandic does not use the same word for every kind of English with.

• við is often used for interaction, contact, dealing, negotiation, opposition, etc.
• með is more like together with, using, or accompanied by

With semja, Icelandic normally says semja við einhvern = negotiate with someone.

So við söluaðilann is the correct choice here.

That ending shows two things:

• it is definite: the seller/dealer
• it is in the accusative singular

The basic noun is söluaðili.

A useful comparison:

• söluaðili = a seller/dealer
• söluaðilinn = the seller/dealer (nominative)
• söluaðilann = the seller/dealer (accusative)

It is accusative here because the preposition við takes the accusative in this use.

It is a compound noun:

• sölu- = sale / selling
• aðili = party, person, participant, entity

So söluaðili literally means something like selling party.

Depending on context, it can be translated as:

• seller
• dealer
• distributor
• retailer

In this sentence, seller or dealer fits well.

Verð is in the accusative, because um takes the accusative.

But verð is a neuter singular noun, and many neuter nouns in Icelandic have the same form in nominative and accusative singular.

So even though the case changes grammatically, the word still looks the same:

• nominative: verð
• accusative: verð

That is very common with neuter nouns.

Yes. A very natural alternative is:

Við semjum við söluaðilann um verð.

That keeps the full verb pattern together:

• semja við einhvern
• um eitthvað

Both versions are understandable and natural. The difference is mostly about focus or style. In your original sentence, um verð comes earlier, which can make the price feel slightly more prominent.

No. Semja has several meanings, so context matters.

Common meanings include:

• negotiate
• compose
• arrange
• draw up
• make an agreement

For example:

• semja lag = compose a song
• semja samning = draw up / make a contract
• semja við einhvern um verð = negotiate a price with someone

So the prepositions and surrounding words are very important for understanding which meaning is intended.