The 2008 version of that bike is £1500 new, I notice he's started the auction off at £800, it might not fetch much more than that but
eBay's funny with bikes, I always manage to get more than I'm expecting. I think the usual rule of thumb for 2nd hand MTBs is deduct 50% after first year and a sliding scale of 10% off the new price for every year (or something like that, no hard and fast rules).
Ultimately it's worth what someone will pay, but most people are looking for something to inspire confidence (good pictures, feedback).
Dave.