BuddyPress.org

The whole point of the GPL is that the code will always be available, even if the developer has given up on it. A more restrictive license might mean that people will not improve on what you have worked on and that your code might just go dormant.

If someone takes your code, improves it a bit and then sells it, the improved code will still have to be GPL. This means that you could then take all the changes made and incorporate it back into your plugin. You wouldn’t even have to buy the improved plugin, just get the code from somewhere. Or just reproduce any new features.