I disagree, or let me say I am of a different opinion. I have bought art directly from an artist at a very low price.
Shortly after I was offered money for the piece that I had bought, more money and I gave half of the profits back to the artist I originally bought the piece from.
Easy. Not sure why you say that is not how it works.
Artist was appreciative and thankful and not disappointed that he could have pocketed everything himself if he would have known.
For me this helped to establish a great communication with the artist and over the years and I was give the chance to buy more stuff from him.
You paint it very negative, but it is all positive. Seller was happy with 800, now he gets what do i know 5k out of nowhere because the buyer found out it is worth much more.
The big difference is, that the seller had a price on the piece !
That is not ripping the seller off !
If he would have been offered 800 from the buyer, and the buyer would have known it is worth much much more, that would have been ripping him of or taking advantage of the seller.
But I guess everyone has their own opinion about it and that is fine. All good.