At night, sneak into someone's house (do not slash the door) and 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. Immediately kill them and run out of the house. You will not be charged with murder and you will be able to buy the house. This will not work every time. If you get caught, either reload the game or use your Guild Seal and teleport away.