the problem is not in the WE_eCart_Definition File at line 465, that is only where the session_start() function is. the problem is at line 5 of the index.php file, look at the error, the output started section:
output started at /Applications/MAMP/htdocs/ProRes_from_Bill/TMP6WT0WISSBX.php:5
tells where to look for the cause of the problem
I'm guessing that you are using Dreamweaver's live preview, that is the cause of the strange TMP6WT0WISSBX.php file name, which is really just your index page. look at line 4 and five of the index page.