Regarding the above example:
The extra £2643.75 the buyer pays is only part of the story. The auction house then takes almost as much again from the seller. This means he - the seller - receives only approx. £13000 of the £15000 hammer price. The auction house itself takes the remainder - meaning it makes approx. £5000 from the sale!
What do they actually DO to earn such huge commission? Polish the paintwork?