Comment your code in a way that makes it quick and easy for a buyer to not only figure out what you've built, but also to allow them to effectively modify your file to their needs. Remember, you're selling to a broad range of technical abilities. At least one comment per function is usually enough, but if it's a complicated concept or there's a lot going on, you should have more.
Make sure your comments are descriptive. If you have a function that runs when the user clicks the left mouse say:
//This is the function that runs when the user clicks the left mouse