I don't understand digital art, but it's the first time I've seen someone like this want to sell their art. What will happen after paying the amount? Will he get an NFT, will the painting be sold and shipped home, or will he own the intellectual rights of that art, or what?
The digital art is a type of art that's created digitally (which means no use of any physical art stuff like pencils, pens, canvas paper, etc,) and that type of art can be delivered to the buyer without any extra cost. This type of deals are done by freelancers on Facebook and other freelancing platforms and they offer such services to the clients. If someone pays the amount then the artist will create the art and send the created art to the buyer.
The art can be used as NFT if the buyer wants to use it as NFT or the buyer may participate in a contest and outsource the art from such freelancers like the OP. This type of painting is digital in nature and there is no physical footprint of such art. Basically the creator owns the intellectual rights of the art but in this case the creator will give complete rights to the buyer. I have been into freelancing platforms and freelancers always offer such type of art services on Fiverr and other freelancing sites.