The idea of making people pay for something that is open sourced puts a horrible taste in my mouth. A lot of the time, people will do as Sofe demonstrated above. People will make such a simple plugin with barely a function. I know a few of the old PocketMiners who left, they would be disgusted in this idea.
I think the solution already exists. If you want to develop and share a plugin not expecting anything in return, use Poggit. If not, do it through your own means. The PMMP team has enough going on, they shouldn't be expected to hold any responsibility over distribution of a plugin, especially when the Pocketmine software itself is free, and people are using that free software and it's API to make money. Yes, it leaves a lot of room for lower quality free plugins and "developers" trying to scam people, but it's not anyone else's job to negate that but your own. An option to donate wouldn't be bad though, as it keeps the motives behind developing plugins the same.
Instead of making a plugin then sell it, I have a completely opposite suggestion: let people invest money then make the plugin. We can have a plugin requests platform where people can write plugin requests and put bounties (in terms of money) on plugin requests. When the developer finishes the plugin, the plugin will become open-source, and he can get the money in the bounties. As long as there is someone moderating (making sure the plugin matches the request before giving the developer the money), this system can work much better than selling plugins, because it minimizes scammers, and plugins can still be public. The only question is: How many people would pay for a plugin that will eventually become public?
What about when multiple people want to meet each bounty, and you have 3 or 4 similar plugins? Does the first one get it? What if the second or third one is higher quality? Does it just become a matter of who is the fastest at making the plugin, or about quality?
Pay to use is a way to fund development Bounties is another way to fund development Regardless of method: We should realize they all serve one purpose, to fund the development of said thing that would not been possible without said motivation or to overcome said opportunity cost. Because nothing is truley free: See PMMP, even tho you can use it for free, it's not really "free". Someone has to put their time and effort into it, They pay it their hard work, bullshit they have to deal with, time and effort, They could have done something else, maybe something that would increase their earnings drastically, or increase their happiness drastically but they choose not to, Just so you can have something nice for "free" of cost. We should be grateful that they dont expect anything in return and say word of thanks to them for doing that. Just because they didnt ask for something, dosent mean they dont deserve it, It just mean they didnt ask for it. And there's nothing wrong to feel that you are entitled of something for your work that you paid in your efforts and the opportunity cost that has incurred on you, there shouldn't be anything wrong with it, code dont write themself right?. It's the means you fund the development, We should create more possible means to fund such development that create a mutually benefited environment for everyone. Personally i think i am quite in favor of bounty simply because it's more open source nature, you're not profiting to sell your plugin, but to make your plugin, thus the users are irrelevant as long as there will be someone there to support you.