At night, sneak into someone's house (do not slash the door); either pick the lock or go in at dusk before the doors close. Go in and kill the people who live there when no one is looking, except them. Quickly kill them and run out of the house. You will not be charged with murder and you will be able to buy the house. Note: This will not always work. If you get caught, either reload the game or use your Guild Seal and teleport away.