"The university wants to buy pens."
Translation:La universidad quiere comprar bolígrafos.
Because the use of the verb infinitive 'comprar' already translates to 'to buy', you don't need the additional 'a' here.
Infinitive verbs used with a conjugated verb (here quiere) default to their unconjugated meaning:
Comprar - to buy.
Quiere comprar - want to buy.