Comma Code - Automate the Boring Stuff
You want to look at str.join
by the last case, len(listVar) is run 4 times, which is very repetitive.  Better to set it to a temporary variable, and check that each time.
Python has some binary convenience  operators for common things like appending.  Instead of listDisp = listDisp + foo use listDisp += foo
instead of calling str() on everything as you build the string, why not call it up front, so it can't be missed somewhere along the way.  Something at the top like stringList = [ str(i) for i in listVar ] will remove the need for all those str() calls sprinkled throughout the code.
Naming:  listVar is a variable.  It doesn't need to have 'var(iable)' in its name.  How about 'inputList'?  Names are important - they're messages to the future (you or whoever reads your code) about what you are/were trying to accomplish.  Similar issue with the name commaCode.
range(len(listVar[:-1])) is the same as range(len(listVar)-1), which is both easier to read, and can take advantage of point 2 and end up written as range(listVarLen-1) which is definitely clearer.


Overall though, for a beginner, not bad!