Thanks for posting, I'll be following that listing with interest.
Personally I don't think it'll sell, but hey, if he gets the asking price then good for him. It's right and proper that timber with an interesting provenance should command a premium, but wide long boards of super prime kilned Oak at a full 100mm thick are easily available at £3500-£4000 a cubic metre. Come down to 50mm thick and super prime kilned Oak is about £2000-2400 a cubic metre. These are top, top prices for magnificent boards. Anyone likely to buy that slab will be fully conversant with the market, they'll know exactly where they can buy similar sized slabs and how much they'd pay, and they'll be fully aware of the problems dealing with a massive piece of still green timber.
So the seller is basically asking £8000 for far less than £2000 of timber, with the £6000 + premium being down to the tree's location. I've made a few pieces recently from Wych Elm felled in the grounds of Hollyrood Palace in Edinburgh, they sold well and the provenance certainly helped clients say yes, but it didn't more than quadruple the price!