I understand your scepicism, but it's genuine.
The seller runs a UK track cycling website and shop and has close ties with Magnus and his wife, Megan. Megan is British (Welsh actually) and Magnus now lives and trains in Wales, hence the UK conection.
The seller was the promoter of Magnus' recent (unsuccessful) attempt at the hour record behind a derny. You could bid on the frame with confidence. Magnus now rides a custom built Bianchi (bianchi are his bike suppliers from his European road team) so that, I assume, is the reason he no longer needs this frame.
His new Bianchi can be seen on the seller's website here:
http://www.trackcycling.net/site/tech/tech.php?id=4

Magnus has been riding a fair bit of track at the new velodrome in Newport, South Wales, over the winter. He's been a regular competitor at their track league aswell as riding some of the higher profile UK track meetings. He also rode the World Cup in Manchester and the 6-day at Rotterdam. It's great to see someone who can win the Paris-Roubaix race mixing it with regular track riders. He's a genuinely nice guy.

It's a bit late now (as the frame has just sold) but here's a picture of Magnus on the frame that was on ebay: http://www.britishcycling.org.uk/new...ackstedt.shtml