Art students & artists have been copying other artists work for centuries. As long as you acknowledge the original source, don't pass it off as your own & don't attempt to financially profit from the work of others, I don't think there's an ethical or legal problem.

Aside from trying to recreate another artist's work as closely as possible, using another artist's work as a springboard for creating variations on a theme is also a well-worn artistic tradition, frequently in the form of homages to artists you admire. In those cases, I think it's perfectly acceptable to sell your work, though always good to acknowledge the source of your inspiration as well...

"Every child is an artist. The problem is how to remain an artist once we grow up." - Picasso