It is possible to sell the original piece
of digital art if you are willing to provide a certificate which says you have relinquished copyright. For that you would presumably want to place a premium charge on the file you send because you, as the originator/artist, will not be able to use the image again except for, say, a reference of your work on your website.
An alternative may be to offer 'Limited Editions' of, say, 50 or 100. You would have to show '1 of 50', '2 of 50', etc, on each one you send out, just as you would when sending out prints. As with the previous example, you would not then be able to commercially use that image again, unless it was used on an object (mug, plate, cushion, T-shirt) that cannot be considered similar.
Once you have sent out a digital art file, however, there are few ways you can effectively police its ongoing use.
This site will allow you to sell digital images so may be worth checking out.
Commission is charged, of course, depending on the size of the image you are offering. This is much less than for prints. I doubt, however, that there is anything in their operating protocols that helps to safeguard your copyright.
It depends on what you want I suppose.
Hope this helps, and good luck!