In the final analysis it's worth what you can get for it. The problem is, as good as a lens it has a limited interest. Your searches on the internet have found this. I gave up trying to get a reasonable (well I thought it was) price for a Nikon 14-24 f2.8. At the time it was pretty much unused but no one wanted to part with £950. Think that was my last price.
Possibly the best place to try and sell it is on
http://www.fourthirdsphoto.com/ They have a buy and sell page and everyone on that site is into 4/3ds. I think they charge these days if your not a paid up member. Presume you've tried here? If not give it a go but don't ask me how much to ask for it.
The logic is you could get the 14-140, which is better for video, for less. You can get that with a GH1 for about what you're asking and wouldn't need an adaptor.
As for trade in it's not good. A friend of mine just traded in an Olympus 7-14 f4 and got £500. This is a desirable 4/3ds lens. Warehouse express have it listed at £1500 ish.
I'm just getting into micro 4/3ds and as much as I'd like this lens there are others that I would like more. There's a 7-14 not selling (As yet) for £700 here, which I would argue is as valuable as yours. So I would say that's about your price-mark. Won't insult you with what I would pay for it as it must be worth more to you unless your quitting 4/3ds
Hope I haven't depressed you too much. I'm just thinking out loud for you.