Do something very important, before you sell them. Unscrew the covers and take them off. Check to be sure that the clock battery (should look like a little blue barrel) hasn't turned into a little white furball and leaked acid all over the motherboard, which in most cases, will destroy it.
If everything is fine there, try and hook it up to a monitor, and take a picture of the screen, being sure that the numbers at the top of the screen are clearly read, so folks can see the amount of memory available and what version of the operating system. Note any cards that are inside too.
