I have lived in south eastern idaho for about 3 years now and have not hunted yet. It is March 7, 2013, and I wanted to know if I can still purchase and hunt mountain line in unit 73?

You can purchase a mountain lion tag for that hunt at any time.  It is a general season hunt so tags are available for sale for 2013. The current season information is copied below.  There will also be a season next fall and your tag will  be valid through December 2013 (unless you harvest before then).   General Any Weapon Regular Tag 8/30/12-3/31/13 Either-sex Mountain Lion Any Weapon Unit 73³