What makes you want to sell individual copies rather than collect a monthly fee by providing it as a hosted service? If it has to be hosted anyway, and unless i

What makes you want to sell individual copies rather than collect a monthly fee by providing it as a hosted service? If it has to be hosted anyway, and unless it's going to be tightly integrated with the buyer's website/server, selling a hosted service might make a lot more sense since you can collect $x/month instead of $30 just once. And you keep your source code, and releasing updates is far simpler.