Download BuycraftPM.zip -> go to DevTools/ -> Create Phar -> upload your "BuycraftPM.zip" -> Download phar -> have fun !
You could also fork it on github, then add it to Poggit in the CI Admin. Poggit will build a new phar file for you every time you commit changes to the plugin source code.
We have builds of the phar file already available. If you login to your Buycraft account and then go to Plugins (https://server.buycraft.net/plugins) you can download the latest released build.