Hi, nice to see someone interested in shader programming, I'm a graphics engineer, my answer may look harsh, but that's from my professional perspective to you:
I'm not going to sugarcoat it: It looks very good, but for a student, not a professional, it really looks like you're learning and got your first piece of art out there, so I don't think it's worth it charging (I wouldn't buy) unless you have a personal emergency or something similar.
To be honest with you, when I was a student I would not be able to do a professional looking black hole, not without feedback about my code, so in your situation I'd not charge so people can comment and help me improve my shader.
Here are a few reasons why and how to improve (without access to your shader):
First, plain and simple: I can find better looking shaders for free (license apart): https://www.shadertoy.com/results?query=black+hole
And those are all animated, while I don't know if yours is because you just uploaded an image, not a video.
However looking better is not the only thing that matters, yours could still look worse and be worth it buying IF you prove that yours runs faster and more efficiently than the ones out there. You see, shader toy's shaders are all ray-cast based, so they are expensive as hell, you could benefit from that if you simply use rasterization with blending and some sort of multi-pass to achieve the same results, there are also ways to improve performance on ray-marching if needed.
So to actually make it a professional work and justify a price tag I think you would need to:
- make it animatable
- upload more images
- upload a video
- add black hole physical effects like the gravitational lensing (If you haven't already because it's not possible to see from your single image).
- Make it faster and more efficient than the ones out there and prove it
- Technical explanation in details about what you did.