Learn Croatian: Ona neće prodati kuću. - She does not want to sell the house.

Exactly. The verb htjeti is irregular. The full form for 'she wants' is hoće. To make it negative, you do not say ne hoće — the words merge into the single word neće (does not want).

Could this sentence also mean 'She will not sell the house'?

Yes! The verb htjeti is used to mean 'to want', but it is also the helper verb used to create the future tense ('will'). Because of this, Ona neće prodati can translate to either 'She does not want to sell' or 'She will not sell'. In real conversations, context tells you which one is meant.

Why is the second verb prodati (to sell) not conjugated?

Because the first verb, neće, is already conjugated to match Ona (She). When you have a helper verb like wanting or being able to do something, the second verb stays in its dictionary form (the infinitive). This works just like English: 'She does not want to sell'.

Why does the word for house change to kuću instead of staying kuća?

The dictionary form is kuća. Because the house is the direct object receiving the action (it is the thing being sold), it takes the accusative case. For feminine nouns ending in -a, that means dropping the -a and adding -u.