There is in practice no first sale doctrine for digital images; the first sale doctrine gives you the right to distribute your copy, not to make your own copies; and you can't transfer a PNG to your friend without making a copy. The first sale doctrine deals with the intersection of copyright with physical copies of a work.
This is why no one is buying and selling digital artworks, except using an artificial concept of ownership - a certificate. The artworld has done it with paper certificates for a long time; NFTs are the same thing.
> This is why no one is buying and selling digital artworks, except using an artificial concept of ownership - a certificate.
You seem confused. A certificate does not grant ownership. The reason nobody is buying and selling digital images is because producing an exact copy of a digital image costs nothing, which means it's irrational to pay for a copy of an image since you can make a copy yourself for free.
This is why no one is buying and selling digital artworks, except using an artificial concept of ownership - a certificate. The artworld has done it with paper certificates for a long time; NFTs are the same thing.