Yahoo, through their developers website, allows websites to use the Yahoo Answers content on their own website through an API. It is legit. I have seen other websites do this.

I think that with you wanting to answer questions using a promotion link back to your site for SEO is a conflict. There are thousands of other people that write answers that do it soley to answer a person's question and not to gain any SEO advantage. If you want to gain link popularity I suggest you look for legit sites that want to link back to you because your content is good.