Uber-collector Charles Saatchi is famously reticent about giving interviews. But Andrew Renton takes him out shopping and for a chat. “I’ve been buying and selling art for 30 years. It’s what I do. There was a time when I was accused of creating a Xanadu, and of hoarding. I can’t win. I don’t mind selling work – even by artists I like. It’s more important to make sure I have the best works.”