Auto Retailer CarMax Trims Outlook  DETROIT (Reuters) - Auto retailer CarMax Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=KMX.N target=/stocks/quickinfo/fullquote"&gt;KMX.N&lt;/A&gt; on  Tuesday cut its second-quarter earnings forecast due to  slower-than-expected used-car sales, sparking a 3.7 percent  drop in its shares.