Fine art pictures are an international currency used by an elite few. Like the banknotes of any country, the pictures that are bought and sold by collectors are worth whatever other collectors are prepared to pay for them. And the traders in this rarified currency are the handful of major auction houses which these days charge both buyers and sellers handsomely for their services.

While the work of some painters or indeed schools of artists in particular periods can fall out of fashion, by and large the value of fine art is a one-way street. It always grows. But paintings and sculptures are vulnerable items. The art world still mourns the exquisite pieces of art that were destroyed in the Second World War, not least those that were vacuumed up from occupied Europe by Nazi leaders, including Hermann Göring. Many of these were lost in bombing raids or deliberately torched by Nazi fanatics shortly before their defeat.

Private collectors often prefer to keep their treasured acquisitions safely in bank vaults much as governments keep the gold bullion that once underpinned the value of many currencies. Publicly-owned art or pieces loaned from private collections are generally displayed in museums with high security and, in the case of the most famous pictures, such as the Mona Lisa, behind protective glass.

Many of the artists who created works that now command tens if not hundreds of millions of dollars, painters like Vincent Van Gogh, died in penury. Only a few attained the heights of wealth and celebrity in their lifetime. Pablo Picasso could doodle a sketch on a paper napkin and give it to a waitress who would later sell it for thousands of dollars. There was indeed the suspicion that Picasso took a perverse pleasure in the slavish acclaim the art establishment gave even his most insignificant daubing.

A widely-celebrated British street artist known only as “Banksy” has taken the modern art world by storm with his well-drafted and not unaccomplished paintings mostly on walls. His fame is the greater because the real identity of the artist has never been revealed. In the Middle East, he is known for his graffiti on the 685-kilometer Israeli-built security wall that he said had “turned Palestine into the world’s largest prison”.

Banksy thinks as little of the elite fine art market as he does of Israel’s oppression of the Palestinians. Last week at Sotheby’s in London, one of his paintings “Girl with a Balloon” had just been sold for over $1.3 million including fees when the picture began to descend through a shredder that the artist had hidden inside the heavy gilt frame. The painter had cameras in the saleroom to film the auction, the painting’s self-destruction and the reaction of those attending the sale. He posted later that he had built in the shredding mechanism “in case the painting was ever sold”.

There were general looks of horror as attendants hurriedly carried away the half-destroyed picture. Only one spectator was captured roaring with laughter and he looked too old to be the anarchic young Banksy. However, this stunt may have rebounded on the street artist. Critics were predicting that such is the craziness of this exclusive world currency, that modern art connoisseurs might actually now value the shredded painting even more highly than the original.