"What do you want to buy her?"
Translation:¿Qué le quieres comprar?
Object pronouns go immediately before the conjugated verb or attached to the end of an infinitive. Both of the following are correct: ¿Qué le quieres comprar? and ¿Qué quieres comprarle?
I think 'la' as you're using it here is a direct object, and 'le' is an indirect object.
The sentence as you've got it seems to be something like What her do you want to buy?"