The 420ex is a good flash and will certainly improve the quality of your shots, especially if you bounce. However, if you care about color balance, be sure the wall is white, or see below.
The easiest way for your wife to get started, though, is to shoot in "Auto" mode with a low resolution. Turn on the camera for shooting, press the "Func" key, and navigate down to the "L" with the quarter-circle next to it. Then press the right arrow to select "S" (640x480). That will give you pretty reasonable Ebay-sized pictures. You might also want to press "*" and select a different resolution. When you've got what you want, press "Func" to get back to shooting.
Don't forget to put it back to "L" before you go to the zoo, though!
If you want to get fancier, I'd suggest you use "P" mode or "Av" mode. In "P" mode, the camera starts like "Auto" but lets you adjust lots of things. In particular, you can select a different white balance. I like to use the custom white balances. Find a piece of pure white paper (or buy a photographic gray card at a good camera store; they're very cheap). Put the paper where you're going to shoot the pictures and make sure it fills the frame (it doesn't need to be in focus). Select the white-balance menu, choose "C1", and press the "*" key. The paper should now look white on the LCD screen. If you're using a 420EX, make sure it is turned on and pointed at the bounce wall when you press the "*".
One last thing about white balance: if you're using outdoor light, it varies with the time of day, so you should do the "*" key thing every time you shoot.
Finally, about advancing to "Av". If you want to control depth of field (DOF), this is the setting to use. The rule is relatively simple: choose "Av" and then turn the command wheel to pick an aperture number. If you want to make the background go out of focus, choose the smallest possible number (2 to 3.5, depending on zoom). If you are having trouble because part of the Ebay item is in focus and part is blurry, choose the biggest number. It's that simple.
BTW, I'm assuming your wife is using a tripod. It would seem to make good sense for her application.