Why is it said that auctions are only the tip of the market?

Because auctions are public and measurable, they get written about — yet they make up only about a tenth of all trade in art. The remaining nine tenths happens privately, off the record and out of the headlines. This is the source of the market's so-called grey area, where the data is missing and true value is harder to establish. A private sale can fetch considerably more than the room, but the world never hears about a record like that.