How much should I sell my camera for?

I'm thinking of selling my Nikon D5100 for a Canon Rebel, along with a few lenses.
The camera is in perfect condition, looks barely used, and has no signs of wear on it. Included with it would be all the accessories that came with it, no more and no less, including the 18-55mm kit lens. The original price was 850 USD.
What would a reasonable price be to sell this for on either Amazon or eBay?

I would go 800 - 600 since you said it has no signs of wear on it.

How old is the camera? What is the shutter count? That count is like the odometer on a car… It's like the camera's mileage reading.

Depending on it's age, perhaps $400.

B&H is selling a used D5100 and 18-55mm kit lens at this very moment for $579.

http://www.bhphotovideo.com/...amera.html

That would be the high end of what you could expect as the camera will at least have been fully checked out by a camera tech, come with free shipping, a B&H money-back guarantee and possibly a 90 day warranty. Unless you are prepared to to match those conditions, you should expect less.

You can also go to eBay and search on completed listings for the camera and lens. Completed listings tell you what they have recently sold for, and that will also give you an idea of the current market.