A watercolour by the great 19th-century American artist Winslow Homer that was rescued from a tip 20 years ago could be sold at auction for as much as £100,000.

Hot tip: Winslow Homer's Children Under a Palm Tree (1885) was found on a tip in Ireland, and is now being offered for sale with an estimate of £100,000

A painting that was rescued from a fly tip in Ireland 20 years ago, and only recognised for what it was on the BBC's Antiques Roadshow last summer, is to be offered by Sotheby's in New York on Thursday with an estimate of £100,000. The lucky vendor was on a fishing trip in Ireland when he found it. When the Roadshow travelled to Earl Spencer's family seat in Althorp Park, the owner joined the queue to have some objects valued. One of the programme's experts, art dealer Philip Mould, immediately saw that the small watercolour of children in oriental dress sitting under a palm tree had some quality. On closer inspection he saw that it was signed "Winslow Homer" – something the owner had missed. Homer is considered one of America's greatest 19th-century artists, and his watercolours have fetched as much as £2 million at auction. As it was an unusual subject for the artist and not one of his most sought-after marine subjects, Mould estimated its value at about £30,000, or £100,000 if it were in better condition, enough to send the owner dashing off to give his family the good news. The work is thought to have been made on a trip to Cuba, and possibly left as a gift to the governor of the Bahamas, Sir Arthur Blake. How it ended in a fly tip in Ireland no one knows.

Last night's documentary on BBC4 about the contemporary art boom was both fun and well timed, beginning with the boom in full swing and ending with the market's capitulation last November. The presenter, Ben Lewis, set out to unravel the mysterious rules that govern the market because he "didn't like what was happening": that art he didn't respect made the most money, and that rich people just got richer by speculating in it. Lewis made some good points, highlighting the extent to which the boom became reliant on credit as it reached its peak. But in his main aim, which seems to have been to expose the role of dealers and collectors in pushing up prices by bidding on the works of artists at auction in whom they have a vested interest, he fell short. If they had inflated values artificially, they would have needed a collaborator to compete against in the bidding process, and this is not demonstrated. That some dealers do bid on those artists (Lewis focused on the Mugrabi family in New York for Warhol and Jay Jopling's White Cube gallery for Damien Hirst), whether for themselves or for clients, is hardly as shocking a revelation as Lewis makes out. During last week's sales in New York, the Mugrabis were still bidding for Warhols and White Cube was supporting its artists, buying two works by Jake and Dinos Chapman; and why not? Not so long ago, dealers rarely supported their artists at auction, and almost every artist dreaded their work being sent to what the painter C R W Nevinson called "the knacker's yard" in case it was either unsold or given away for a ludicrous price. They must be sorry to see the boom end.