Setting aside works whose rights are in dispute, have been sold or whose creators are long dead, can you elaborate on this in light of what we know about, for example, Hollywood accounting? If you want to put money in the hands of your favorite living artists, there are more efficient ways than buying their work through a middle man and hoping they live up to their end of the bargain. Buy merch directly through them, for example. More independent creative types tend to have things like PayPal for this as well...

https://en.wikipedia.org/wiki/Hollywood_accounting