selling,organizngby is a preposition, followed by the verb -ing or a noun, and is a conjunction, indicating that the preceding and following phrases are in parallel relationship, so the second space is also in the form of ing. This sentence can actually be split into two short sentences UNICEF raises money by selling

Christmas cards.

UNICEF raises money by organizing other fund-rasing activities.

UNICEF raises funds by organizing other fund-raising events.