It was not surprising that for most of the day the man selling black and white Banksy prints in New York got no takers. Coming from a pop-up stall in Central Park, among many others selling cheap tourist souvenirs, they were outrageously expensive at $60 each. Gift shops were selling artists' posters, greeting cards, mugs and coasters for a fraction of the price.

A man who had bought two canvases from the stall, proved, by mid-afternoon, to be only the second customer for the prints.

But this buyer now stands to make a small fortune. The pictures have been authenticated by Banksy; they are going to auction at Bonham's, in London, and they could fetch up to £120,000.

From the pile of canvases stacked up on the trestle table, and hung from its metal framework, the buyer had selected Kids on Guns – two sweet little children standing on a hillock of guns and bombs – and Pooh Bear, a version of AA Milne's winsome creation sitting weeping under a tree, honey pot (labelled with a dollar sign) discarded and his foot stuck in a bear trap.

The two pictures will be in Bonham's contemporary art sale next month, and the estimates are that they will make up to £70,000 and £50,000 respectively.

They were bought during Banksy's visit to New York last year, when he played hide and seek with both the authorities and fans. He created street art, including a shoe signer polishing Ronald McDonald's boots, the location of which was revealed daily before admirers rushed to try to take selfies or the authorities went in to destroy the work. On one day the art site's message was: "Today's art has been cancelled due to police activity."

The Central Park stall was authenticated the day after the sale, with a message on Banksey's website saying: "Yesterday I set up a stall in the park selling 100% authentic original signed Banksy canvases. For $60 each." The website also warned: "Please note: this was a one off. The stall will not be there again today." Perhaps he was anticipating an entrepreneur stepping in to fill the sudden demand for art in the park.

Gareth Williams, head of contemporary art at Bonham's, said the Central Park stall was a coup. "The fact that his paintings were original and were being offered at a tiny fraction of their true retail value, raises real questions about the perception of worth and the nature of art as commodity within the marketplace – something that the artist must be acutely aware of."