I sent a message to the seller asking the same thing, and they thought it was actual Hasbro art at first, so they took down the item. If you see any other pieces of your art on there, just politely let them know.
Well, I use Photoshop, and I use the base in my gallery: [link] and if I can help it, I find good coloring pages and arrange them appropriately. Then I color it to look nice. Here's my tutorial for coloring: [link]
Again, love some the little details you put in this piece! I also like how you colored her tail. The only thing I would suggest is maybe pushing back the saturation or colors of the background blues a little bit so Princess Luna could really pop!