"Buying her a trumpet was not a good idea."
Translation:לקנות לה חצוצרה לא היה רעיון טוב.
Non-native alert. I think that this is simply a preference in Hebrew to use the infinitive over the gerund.
If it is possible to use the gerund, I suspect it would need to modify to קניית because of the חצוצרה that follows (although perhaps של could be used instead). But can one split the construct form??? And where should the לה go?
On balance, I think that the use of the infinitive keeps things simple!!!