Just did this one last week on the same topic and almost price. http://nikonrumors.com/forum/topic.php?id=2548
For buying used, I look at the condition and pay note of of the rubber grips. If they are faded or the finish is really warn, I know it was used allot and I tend to stay away - since it had more opportunities to get banged up.
When and where bought and receipts don't mean a thing to me. It was new once and used now - what matters is what happened to it in between.
It sounds like you don't really know if you need the lens or not and from what you wrote, it sounds like you are not sure what to do with $1k for camera gear - Is this correct? (BTW-35mm 1.8 is a keeper.)
If you have $1k for lenses there could (stress could) be more beneficial one's than the 17-55mm which is a big lens and on a D40 is huge. For a small kit like you have, you may like the 18-200mm to combine your other two lenses, and sell them for your camera upgrade. If you are not opposed to buying used and are upgrading to a camera with a AF drive, for a grand and selling a lens or two, you could easily pick up a 50mm, 60mm Macro, 70-300vr and maybe even a 16-85mm or the 18-200mm. Just a thought but you will have to answer that for yourself.
When I look at purchasing lenses, cameras, etc. I always run myself through the following questions. If what I'm looking at lines up with that particular item then I go for it.
Here are the questions I run myself through:
Who or What is your subject?
When are you shooting?
Where are you shooting?
Why are you shooting X?
and How do you want it to look?
