What to check for before buying an used Nikon D3100?

I want to buy an used Nikon D3100 body. Since I'm relative new to DSLR, can someone tell me what to check for in an used camera before buying it?

Make sure it works. Find out why it's being sold. Those cameras should work forever, so why sell it?

Take a picture, be sure you can see it in the review display. Take the SD card out and make sure a computer can read it. Make sure that autofocus works correctly. Take pictures of items at different focal lengths using autofocus, but sure each is in focus. If the camera was dropped, it can screw up the autofocus mechanics.

Take the lens off an look inside the body - look for signs of water damage. Is there any mold/mildew inside? If yes, don't buy it.

Be sure you get the battery charger. It's a separate device that plugs into the wall w/battery attached.

Same as any other DSLR.
Number of shutter actuations (life of its shutter is how many - you could look that up?)
If all the controls work.
If all the doors stay shut.
If the battery stays in.
If the lens mount and contacts look OK.
If the lens is in good condition and everything works in AP mode
If there's a warranty.
If there's a return agreement in place.

Take a lens and memory card with you.

Make sure that the seller has a fully charged battery in the camera before they meet with you to exchange your money for their camera. Insert the memory card and attach your lens. Take some shots and make sure that the camera is really working.

You also need to be sure that you get all the accessories that came with the camera when new.

EN-EL14 Battery
MH-24 Battery Charger
DK-5 Eyepiece Cap
DK-20 Rubber Eyecap
AN-DC3 Camera Strap
BS-1 Accessory Shoe Cover
BF-1B Body Cap
ViewNX 2 CD-ROM