There should be a man with a sandwich board standing outside Sotheby's today. The Bond Street auction house seems to be starting a grand closing-down sale. Everything must go from the vast Damien Hirst emporium. Form an orderly queue - it's your last chance to invest.

Beautiful Inside My Head Forever is certainly a landmark exhibition. Hirst has stated that while he will continue to make pieces that preserve animals in formaldehyde, the butterfly and spin paintings that he is now putting on the market are the last of their sort.

He is taking an unprecedented gamble. In a move that could change the face of art dealing, he is cutting out the middleman and becoming the first artist ever to create an exhibition specifically for an important auction house. It's a risky decision. Without a pin-striped dealer to line up the right clients, it could all go very wrong on the night.

The dealer is not necessarily just creaming in the cash. He can play an important role in art history. Think of the connoisseur Daniel-Henry Kahnweiler, for instance, who in Paris at the beginning of the 20th century collected and traded Cubist works. “What would have become of us if he hadn't had business sense?” asked Picasso, who was all but destitute before being discovered by Kahnweiler.

Contemporary dealers can also have a huge influence. Without Charles Saatchi, Brit Art may never have existed. But, a bit like those old millionaires who marry their secretaries, the relationship can hit the rocks. Dealers get greedy - as much as a 60per cent cut of the sale price is not uncommon.

Discontented creatives have long been exploring alternatives. They get a bit on the side by selling straight from the studio, or they play the field, scampering from gallery to gallery. Some go it alone, hiring a shop front and putting on their own show.

But, as investment capital floods into the art market, the sharks are closing in. The auction houses that once dealt with a secondary market have been steadily encroaching on the territory of the contemporary gallerist. It is Christie's, for instance, which owns and backs the many international branches of the Haunch of Venison gallery.

Will Hirst's latest act of bravura pay off? It certainly looks like it at first glance. This show is an extravaganza. Hirst's career reaches a glittering high point in a vast display that takes over the entirety of Sotheby's.

It is incredibly striking. This most audacious of artists pushes his vision to its glitziest limits in his new works. Gold gleams, diamonds sparkle, the iridescent wings of butterflies dance. It's hard not to be dazzled.

Hirst never does anything by halves, unless he's slicing animals. Expect a lot of everything - 223 lots, to be precise - in what feels a bit like a huge warehouse clearout.

In two weeks' time this show will be picked apart and put under the hammer, piece by piece, in an unprecedented two-day sale dedicated to a single contemporary artist.

What will the buyers who are so keenly competing for prime saleroom places be getting? A piece of art history preserved in formaldehyde or a lump of mass-produced hubris in a terrible pickle?

With reports that unsold Hirst pieces are stacking up in the basements of his usual dealers (among them the notorious £50 million platinum skull that was bought by a consortium of investors including Hirst and the White Cube director, Jay Jopling) a lot rides on the success of this sale. There are several star pieces. The Golden Calf, a vitrine in which a huge charolais bull lowers its head patiently to the weight of a pharaonic head-dress as he floats weightlessly on gold-slippered feet, has a sublime beauty.

Hirst's animals in formaldehyde - and there are several in this show, from a tiny winged piglet through a zebra to a unicorn - may be familiar but they never feel formulaic. They never fail to stir a sense of the ineffable sadness of existence, of the patient dignity with which we bear the expectations and sufferings and desecrations of life - and of death.

His sharks, those iconic cruisers of the conceptual scene, trap a sense of futility in a tank. They have a haunting power.

The butterfly paintings, in which myriad shimmering wings create luminous stained-glass patterns or glow against the darkness of a diamond-studded abyss, are as lovely as they are eloquent.

Hirst's iconic meditations on death are the pieces that will probably attract buyers. They are the ones with the lasting impact. If The Golden Calf sells above its estimate of £8m-12m it will top previous auction records for Hirst.

But the more decorative works - the spot paintings, for instance, seem mere commodities that have been mass produced to fob off a hungry market. They may be brands as recognisable in their way as a Smarties packet, but they are just as hollow. They feel intrinsically valueless - and even a background of gold can do no further good.

They may sell. Hirst may not have a middleman but he has the tradesman's oldest trick up his sleeve. This, he is telling us, is the last chance to buy.

I expect that, just like those endless closing-down sales, the end will never come. But if the tactic works for now, why should that worry him? Hirst's talent has always been as much for commerce as for creativity.

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