Thnx Ian.....
Though what I have seen is some adding a certificate of authenticity, adding small round authentic stickers on the back of the backing board....things lime that!

In my experience, Karl, it makes little difference as to the providing certificate of authenticity, unless you are a recognised artist and paying a premium for the print, if some one likes your picture you will find that’s why they buy it, it matters not whether you used film or digital, if the viewer likes it enough and you can produce a good print, they will buy it, the main thing is to sign the print.
I would think they have to ask that by selling other people’s art works from their web site, they are acting on behalf of the artist, selling your own work from your own web site is a little different
I think if you went down the that route then embossing is a better way, unique to the artist, wont be copied etc
By 'embossing' you mean a physical stamp, a bit like the old wax seal?
(And not something added in Photoshop.)