Generally when buying from Amazon you can be sure that the product is is good, working condition. I personally wouldn't buy anything second hand without being able to try it first.
You shouldn't have any problems with the warranty. Regarding the picture count try to get a picture taken by the camera and then read its EXIF date using a program such as Opanda IExif which will tell you the number that the picture is (ex. If the picture is 203, it's the 203rd picture taken by the camera).
If I were you, I'd personally pay the extra bit of money to go with a brand new D7000, you don't have the hassle of all of the precautions that you need to take with a second hand body, plus it's difficult to test the body considering that it's an online sale.
Good luck shopping!
Gear: 7 Nikon Nikkor AI-S and AF-S lenses, SB-700 flash, Nikon D7000, Nikon FM, variety of accessories
"There are no rules for good photographs, there are only good photographs."
- Ansel Adams