Of all his crafty sales tactics, the one used to greatest effect by Christie’s 52-year-old global president – the auctioneer on top of the wave of wealth in the world’s art market – is the query: “Are you sure?” He asks it as one of the last two bidders in an auction drops out, threatening to finish it. As he halts the action for a few seconds – what he calls “the auctioneer’s pause” – pressure builds on the reluctant bidder.

Perhaps he will deploy it on Monday, as he stands in Christie’s saleroom in New York and attempts to sell a 1917 Modigliani nude for more than $100m at the start of what Christie’s and its biggest rival Sotheby’s hope will be another record-setting week of art auctions.

It is a deft inquiry for it is impossible to dismiss – of course the bidder is unsure. No one is certain because no one knows what a work of art is worth – perhaps a few dollars more, or a few million.

Short, fun profile of Christie's top auctioneer Jussi Pylkkanen.

Auctions are unusual in the 21st century – most things, even luxury items such as watches and clothes, sell at fixed prices although there is some room to haggle. Even many items on eBay, the electronic platform, are sold at fixed prices. In The Dynamics of Auction, Christian Heath, a professor of work at King’s College, London, describes them as “a somewhat anachronistic method of selling goods, more common perhaps to traditional agrarian societies than post-industrial capitalism.”

They are still used for art because every painting is different and has no intrinsic value – it does not yield anything and the cost of manufacture is usually tiny. They are also a good way to get high prices – when buyers compete against a deadline, they behave differently. The desire not only to acquire it but to beat others causes what Deepak Malhotra, a Harvard professor, terms the “emotional arousal” of auctions.