How do we know the Mona Lisa only has one copy and is unique? or any other piece of art for that matter?
Regarding the painting on your wall: are you the author of the painting? Do you have copyright over the art and permission to re-sell it? If you are not authorised to re-sell it, then that is a copyright issue. Or you could take a photo of it and digitally alter it sufficiently to create a new piece of art, if it wasn't your art to begin with.
If you take a photo of the painting and sell a digital copy, you list how many copies you have to sell on OpenSea and that is your unique counter. You are selling a digital copy of the art, not the art itself. Unless you include the tangible art in the contract you are selling, then that's a different story, and you would probably have to send the art piece to your buyer in whatever way you agree. I know an artist who paints original paintings, photographs the paintings, then significantly edits the photos digitally. Those digitally edited paintings are then printed, mounted, and sold as limited edition giclee prints. The original painting is just part of the process of creation. How does anyone actually know that he is only making one printed copy of each artwork? I dunno, scout's honour. Just like any artistic artwork with limited editions.