Learn Croatian: Moji kolege žele prodati svoje dionice. - My colleagues want to sell their stocks.

The word kolega refers to a male but ends in -a. In the plural, the noun itself takes the ending -e to become kolege. However, because it is inherently a masculine noun, any adjectives or pronouns attached to it must be in the masculine plural form, which is moji.

Why use svoje instead of njihove for their?

You must use the reflexive possessive pronoun svoje when the subject of the sentence owns the object. Since the colleagues are the subject and they are selling their own stocks, you use svoje. If you used njihove, it would mean they are selling some other group's stocks.

What case is svoje dionice in here?

It is in the accusative case because it is the direct object of the verb prodati (to sell). For feminine nouns like dionica, the accusative plural ends in -e, which happens to look exactly the same as the nominative plural.

Could I say žele da prodaju instead of žele prodati?

When the subject who wants something is the same as the subject doing the action, Croatian strongly prefers using the infinitive, so žele prodati is the natural choice. Using da plus the present tense is understood but is much more characteristic of Serbian.