[ForrestN]

I don't see the relevance of app stores to the sale of artworks...?

Auctions take a small cut because they are resellers of art. The artist doesn't get paid when a work sells at auction, the previous owner does.

Art is usually sold by galleries, and the standard cut there is 50%. On top of that, artist and gallery often share discounts, so a work that is sold for a 20% discount means the artist will only see 40% of the total price. For artists, this will be a pleasant surprise rather than a steep expense.