Are you working with PHP, if so you can have the target list post as an array of values. You can then use the implode function to create a string from the array.
You would do this by first adding in square brackets just after the name of the element like this:
Then after the form is submitted you can refer to the array and implode it like this:
$myvalues = implode(", ", $_POST['thename']);
This would create a comma separated list of the values that were submitted from the select list.