For sale sings posted outside houses are usually made of a material called Vinyl. Vinyl is a synthetic man made material and is a specific type of plastic. Vinyl's properties made it ideal for a 'For Sale' sign.
THE GOVERNMENT!
To inform the public.
On the wall
Regulatory purposes.
Their clothes, their houses, and their pride.
It depends on the laws of your state
If the reduce speed signs are posted, yes.
There are. In countries that drive on the left, that is.
In the related links box below, I posted the information.
Look in a local paper shop or just a standard local shop. Housing agents and things similar, look out for some signs reading "For sale" outside of the houses .
At a four way stop
If someone does not invite you to, expect you on, or want you on their property, yes.