Ankit,
Its a toss-up with your plugin. It would work great as both a WordPress plugin or as a standalone self hosted script. I believe I would have more options with a self hosted script though. Some existing sites do not use WordPress, so you would be able to reach more users of your script by developing a stable self hosted version. I would even go so far as to suggest you do both. A self hosted option with the ability to brand it would be great!
Please keep me updated
Harvey
I generally prefer self hosted scripts for any client. Your solution was the absolute perfect solution. I do prefer WordPress plugins more than anything. I’d rather give the client and the client’s customer one central place to make all of their transactions rather than use several systems with separate logins and UI/UX designs.