The first thing you want to do is definitely to split the two comma separated lists into arrays of strings. Assume that they are formatted fairly reasonably, you can do this with
possible_values = possible_string.split(/,\s?/) //split on commas with a possible space
selected_values = selected_string.split(/,\s?/)
If you are willing to use outside libraries, underscore.js has a perfect function for this. The operation you are describing is the set difference operator, which is the difference function in underscore.
The result you want is the return value of calling
_.difference(possible_values, selected_values)
