He is being taken into custody at a south London police station after being arrested on suspicion of a Computer Misuse Act offence.
The seller, who hid their identity by calling themselves the"Crafty Cockney" and "Mas", contacted the press and said they had 3000 private photographs from the author's Apple iCloud account.
They reportedly demanded at least £50,000 and said: "This isnt [sic] an auction it will be a simple process of the highest bid."
The seller put them "on sale" and wanted to sell them within 48 hours as a 4pm on September 23.
They added: "Due to current climate on privacy laws in the UK I'm intending a quick US sale but would at least give you a heads-up."
A source close to Middleton confirmed her lawyers had been told and that they would be speaking to the police.
Middleton, who is the younger sister of the Duchess of Cambridge, reportedly had a collection of pictures of parties, royal children and wedding dresses.
The seller, whose gender is not known, had obtained the images and wanted to speak to journalists.
They added: "This is obviously a vast source of info and pictures that I neither want nor intend to keep in my possession very long."
The hacker initially wanted to sell the images to a magazine based in Britain or the US but added they would return the photographs to the owner once payment was made.
It comes after a number of celebrities had their private online accounts hacked, including American actress Jennifer Lawrence, 26.
Model Kate Upton, 24, also was a victim of an iCloud theft in 2014.
It is thought that more than 100 celebrities fell victim to the hack, which resulted in revealing pictures of them being posted online.
