Borders Posts Third Quarter Loss  SAN FRANCISCO (Reuters) - Book retailer Borders Group Inc.  &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=BGP.N target=/stocks/quickinfo/fullquote"&gt;BGP.N&lt;/A&gt; on Tuesday posted a quarterly loss versus a year-ago  profit due to sluggish store traffic and a slowdown in sales of  bestsellers.