The method Villager#setProfession() sets the villager's profession but it's appearance won't change and he won't offer any trades even if you add some by using Villager#setRecipes().

This could happen because a villager needs a "working block" in order to change it's profession "naturally". By setting the profession using the method Villager#setProfession() he won't get this block, so his appearance won't change and he won't generate (or accept manually added)  offers.