Every night at the Saratoga Yearling Sales, crowds gather outside the glass-walled pavilion like waifs pressing their noses against a rich man's window.

The spectators come and stare, not because of a passionate interest in the economics of the thoroughbred breeding industry, but because the sales offer a rare human spectacle. Seldom do the masses get to see so much of America's social elite congregated under one roof. And seldom does anyone get to watch them do in public what they usually do in private: spend money.

The yearling sales are one of the most dazzling displays of conspicious consumption. A wealthy man can spend hundreds of thousands of dollars in a minute or two in many ways - he can do it simply by dialing his stockbroker - but where else can he do it with a rapt audience?

Like most activities of the very rich, the yearling game is played with decorum, with unspoken ground rules that insure civility.

Potential buyers do their shopping in the mornings, inspecting yearlings at the sales barns. It is practically impossible to tell much about the future racing capabilities of these plump little butterballs who have been fed and groomed to look as dazzling as possible for this sale. But buyers must try anyway and inspect them with care, discoursing on their pasterns and their cannon bones and the slope of their shoulders.

A sales customer who could never distinguish one horse from another would utter the words, "What hocks!" whenever he saw a yearling, using inflections that suggested anything from awe to shocked disapproval, and thus was accepted as an expert on conformation. Nobody wants to look ill-informed or imprudent when he spends money before his peers.

When a yearling enters the sales ring, the auctioneer begins his rapid fire chant, and tuxedoed bid-takers yell "Hip!" to indicate that a bid has been made. But to most onlookers the bids seem to be coming from ghosts. The prospective buyers don't yell, don't wave. They establish eye contact with a bid-taker and make their bids with a nod of the head or a similarly undramatic gesture.

This custom makes novices in the pavilion almost afraid to move, and sometimes with good reason. On Tuesday night a man waved to a friend on the other side of the aisle, only to find that his wave had been construed as a $107,000 offer for a colt by L'Enjoleur.

When the gavel falls and the auction has ended, the winning bidder is presented with a sales slip, which he signs with a bored, world-weary air, while his beautiful blond wife looks on with admiration. They they adjourn to the next-door bar, the Spuyten Duyvil, and drink to the future success of their new acquisition.

One middle-class spectator, who comes to Saratoga every year, watched the sales here the other night and said plaintively, "This is the only time of the year that I'm conscious of the fact that I'm not rich." Indeed, people who have never had any great lust for wealth may feel sharp pangs of envy when they watch the people who play the yearling game. It is irresistibly glamorous.

Some day I'd like to have a furbedecked leggy blond looking on breathlessly as I sign the tab for a $100,000 yearling. Or I'd happily settle to participate in a conversation I overheard at the Spuyten Duyvil the other night.

Sweet Young Thing: What brings you to this country?

Man: I'm looking this year and buying next year.

SYT: What are you looking for?

Man: Horses and yachts.

The style of life on display at Saratoga is so expensive, so elusive, that even mere millionaires cannot afford it. The rest of us cannot even dream of it. We have to be resigned to be on the outside looking in, with our noses pressed to the windows of the sales pavilion. CAPTION: Picture, Hip 24, a chestnut colt, is sold for $205,000 to newspaper publisher John Knight's Fourth Estate Stable at Saratoga Yearling Sales. AP