It doesn't sound too bad, although I'm assuming you mean that you would create a piece and then first person to buy it gets it "exclusive." This can work but it might not be what people are looking for.
Another option you could do is have a contact option where someone who is looking for exclusive art can pay to have you do it to their specifications for a higher price than just a piece you created that has an exclusive license.
Either can work and I'd think it'd be awesome if you did both, to give a more "budget" friendly option or if they just like the art style as your doing it now, and to give the option to those who would be willing to spend more for a unique piece.