Yahoo claims ownership?
how is it legally possible that yahoo can claim ownership and/or copyright of my literary works that I choose to post?
By submission am I transferring my rights?
Yes, you are giving up the rights to what you write upon posting them. It's like that at a great many public forums and boards, IMDb among them.
Yes it be part of the T&Cs Read the agreement you agreed to when you signed on. It will tell you everything.