Seems a pretty standard agreement but grifkin may wonder whether the painters employed to make copies of the artist's work, or their employers, are paying anything to the company that has got the reproduction rights

They might do but my guess is that they don't. Yet they have not, presumably, been injuncted or sued for damages. And the reason for that is likely to be either a) nobody can find a clear precedent or law that says that an artist who paints a new painting which is a copy of Mr Kincade's work is liable to Mr Kincade's rights holders. It may be treated as though it were an original work by the copyist, notwithstanding that it is a more or less faithful imitation of an original.That may sound unjust or unfair, but that's quite an attractive argument to raise in a copyright case

And it may not be worth the time and money pursuing the copyist though first instance and appeal(s) just to get a definitive answer: there's always a risk that you'll lose at some stage. You don't get popular with lay clients if that happens
or
b) whatever his rights the loss occasioned by this activity is not such as to be worth the cost and effort of pursuing. That might be just that, even if the copyist is in the USA, or, more probably, that the copyist is beyond the jurisdiction. In the latter case it is likely be impossible to get or enforce any judgment.
It would be difficult to pursue E-Bay because they are merely providing a service and not 'publishing' the offending work.