I agree with this, but can someone really be held liable for something like this? I assume the painting did not come with a contract that states the artist/seller must always claim the work, from now to forever?
Not claiming his work is different from actively denying having made it. You don't need a contract to be sued for causing damages by knowingly making false claims.
Assuming he's actually the author, he's in a way "defaming the painting".
Assuming he's actually the author, he's in a way "defaming the painting".