It's not about selling. As long as you give the amxx you must also give the sma file. Even on my VPS I upload both the sma and amxx files of my private plug-ins and not just the .amxx file. But in your situation ou migh just have to find a better hosting company like NFO and host your game servers there. This way you won't ahve to worry about your plug-ins being stealed. And if you sell a plug-in, you must include the .sma file as well to the buyer.