Radical thought: the original owner of a “sleeper” should be compensated in future sales of the work of art, which they sold for far less that its true value. That’s an idea which, when I’ve bandied it around, has been met with open-mouthed incredulity from art market players. Anathema! Sacrilege! Absurd! No—it makes perfect sense. Here’s why.As any Old Masters dealer will tell you, the Old Masters market is built on the buying and selling of sleepers. Off the dealers go to this or that auction, to buy that “Italian school” painting, which might, under all that grime and dark varnish, actually be “workshop of Botticelli” or “follower of Caravaggio” or, even better, might bear the brushstrokes of the master.The art market celebrates the discovery of sleepers: as the way new masterpieces are brought to public view, as a driver of art history, and as the ultimate expression of the skill of a “sleeper hunter” dealer. There can also be a lot of money in it. In June 2022, Colnaghi Gallery proudly unveiled a newly discovered Annunciation by the 17th-century Italian painter Caterina Angela Pierozzi. The seller had bought it for only €7,800 at Millon auction house. Colnaghi put a six-figure price on it. In September 2022, a dealer bought an unrecognised Artemisia Gentileschi at a Danish auction house for £30,000 and sold it less than a year later at Christie’s for £1.9m.And yet these are stories with losers as well as winners—though you rarely hear from the victims. The original owner of the Pierozzi, whose identity we do not know, would have made only a little over €6,500.The Salvator Mundi is the most famous and extreme example of a sleeper in the history of art. In 2005, the owners, the Hendry family, who’d been looking after it since 1958, sent their picture to a local auction house where it was sold for $1,100 including fees. The Hendrys received only $700. Twelve years later the picture became the world’s most expensive painting when it was sold at Sotheby’s for $450m—642,857 times what the Hendrys had received. That was its third sale, having in 2013 sold for first $83m and then $127.5m. Yet the family had tried to get their unrecognised Leonardo da Vinci correctly valued. A young member of the clan, herself an artist and Renaissance enthusiast, took a photograph of the painting and sent it to Christie’s, asking if it wanted to sell it. Christie’s sent a specialist, who picked up a few family heirlooms for auction but passed on the Salvator Mundi; it was subsequently picked up by a local auctioneer in New Orleans. Christie’s cannot be held accountable for passing on the picture. It has the right to decide, rightly or wrongly, what it includes in its auctions. But was this fair commercially to the family?Leonardo da Vinci’s Salvator Mundi (around 1500) Selling a valuable heirloom for a pittance can have a severe effect on the mental health and family relations of the sellers. It is rare for sleeper-owners to speak out, but I know of one case where a young co-owner of a sleeper was living in a trailer after her picture was sold for hundreds of millions.It would not be difficult to draw up a set of rules for Sleeper Resale Rights (SRRs). The original owners would need to prove that they had owned the painting for a considerable length of time, such as ten years, or that it was an heirloom, which had been in the family for longer than one generation. The percentages could imitate those for artists (under Artists Resale Rights laws, the artist gets 0.25% to 4% of the price of the resold work). But unlike those for artists, they should be calculated from a figure that is the new sale price minus what the owners sold it for (so they aren’t simply enjoying two paydays). It would seem fair to limit the period that SRRs were payable to 100 years.There is nothing wrong with making large profits. Yet in recent decades we have developed a vocabulary with which to censure ostensibly legal but exploitative behaviour in the marketplace. Corporate practices may be “predatory”, their clients may be “vulnerable”. The information relationships could be “asymmetrical”. The stronger party has a “duty of care” to the weaker. These adjectives are a perfect fit for the rip-off sleeper business. It is high time for predatory sleeper hunters to recognise that they have an asymmetrical relationship to vulnerable individuals and families who find themselves pressured by circumstance to sell off their treasured heirlooms. The art market must give due consideration to its duty of care and work out a system to fairly compensate the original owners of sleepers.• Ben Lewis is an art critic, historian, author and broadcaster. He wrote The Last Leonardo: The Secret Lives of the World’s Most Expensive Painting