Ben have a look on some of the used equipment websites and see what they are making, seen as low as £585 and as high as £700, depends on condition and how quick you want to sell it.
I they're going for anywhere between 585 and 700 according to martyn, and yours is in "mint" condition as I'd expect, what would lead you to believe that you're only going to get 500 for it? He was talking used, not new... 700 for a mint 17-55 sounds perhaps a trifle high, but you might be able to pull it off. Just gotta find someone who cares an extra 30-50 pounds about perfect condition.
