Each one should tell you which guild would be interested in them. You can sell them to any of the NPCs that give the lower level writs; I will list the NPC that I usually sell to, they are usually the easiest NPC to find.
For the scout guild you need to go to Elddar Grove near the big tree in the middle (NPC Wanderer Greencoast).
For the mage guild you need to go to the mage tower in South Qeynos (NPC Pupil Adept Wazzlefop, he's on the main level when you walk in, not necessary to take any of the colored warp thingies).
For the priest guild you need to go to the Temple of Life in North Qeynos (NPC Caretaker Nogfizzle, right outside).
For the fighter guild you also need to go to North Qeynos, they are standing outside the castle (NPC Guard Williamson).
Hope that helps!