Creating a product that already exists

Hey guys!
I've been working on a desktop application over the past few months as a way to learn python and the QT gui framework. I never had any plans (and still don't really) to make the app public but I was wondering if it's still possible to market and sell a software product that already exists or could it get me into trouble.
Is it possible I'd be infringing on a patent or copyright (or maybe some other legal issue)? If so, how would one go about finding out if they are.

Please feel free to share any stories where you or someone you know ran into similar issues!

Thanks!