There's no way to compile this project. Not even all required jars are provided with the project.

That is indeed a mistake and I've tried to correct it. Ideally the root of the repository would need to move from the source directory to the project directory so the lib and nbproject directories are added to github.
Haven't been able to do that without losing history though.
So for now you can get the dependencies from here: https://dl.dropboxusercontent.com/u/4109728/SignShopLibs.7z
A copy of spigot, which SignShop also depends on, is obviously not included in the zip.

You could turn the project into a maven project. You can customize the location of the source and resources.