Without much tinkering virtually any digital camera or scanner will do a good job for eBay sellers. That's because the depth of detail necessary for displaying images on a computer screen is much less than for printing them. You don't need an expensive camera or scanner and usually you can depend on the device's automatic settings to get good results.
With scanners the most important thing is getting the item perfectly flat on the scanner's bed. For cameras it's having enough light.
The truth is 99% of the effort required to produce good digital images, regardless of the device being used, is giving the device what it needs to do a good job. Along with good lighting such things as holding the camera perfectly still or using a tripod, shooting from the right angle to reduce the impact of shadows, using a back drop that enhances the subject (or at least doesn't distract) are all usually more critical than mastering advanced settings.
That said, here are a few things you should know about your camera and/or scanner:
Don't automatically choose the highest quality setting
Digital cameras and scanners are designed primarily to produce images for printing so what the device refers to as "quality" in its menu of settings relates almost entirely to printed output. Because you will not be using the images for printing you should not automatically assume the highest quality settings are best to use.
As an example, let's say we want to produce a 640x480 (640 pixels wide by 480 pixels high) image. If we set our camera or scanner to its highest quality setting it will produce an image very much larger than 640x480 pixels when viewed on a computer screen. That's because higher quality settings, expressed in terms of megapixels for cameras and resolution or DPI for scanners, relate to density rather than size. Basically, the higher the quality setting, the more dots are being squeezed into a square inch of the printed result to produce a more detailed print.
But I want to show very high detail so my photos must be very large, right?
Not necessarily! Use the cutaway technique instead. Rather than displaying the entire item huge, pick out a few of the most important parts. See the tutorial: The Big Four: Crop, Cutaway, Resample, Compress
With computers, however, there isn't a way to squeeze more or less dots into a square inch of screen space. Every one dot translates directly to one pixel. So, without exception, the greater the density of image produced by a camera or scanner the larger it will appear on computer screens. Thus, the only way to make an image appear smaller on a computer screen is for it to be less dense. Typically the camera or scanner itself has better ways to produce a smaller (less dense) image than we do using a photo or graphics editor so, as odd as it might sound, it is usually better to choose the lowest quality setting available with the device.
It is likely even the lowest "quality" setting on a scanner or camera will still produce a larger image than our 640x480 pixel example. At least then the degree of adjustment will be minimized and so typically any deleterious effects on the image display quality. Along the way you will also benefit by having less memory and disk space consumed which, in turn, results in faster uploading times.
Macro vs. Normal Focus
Some cameras have what is called a macro focus feature that allows the camera to be held within inches of the item being photographed and still be focused. Rather than using zoom to get extreme close ups use the macro feature if you have it. But, remember to turn it off when shooting from a normal distance.
Optical Zoom and Digital Zoom
There are two kinds of zoom - optical and digital. Optical zoom is produced by a lens; digital is calculated by the camera via a process called interpolation. Optical is reality; digital is an estimation of what reality would look like larger. Obviously then you should try to get by using zoom only up to the extent it is optical. For example, you may have a camera with 3X optical, 10X digital. For the most accurate result you should consider getting closer if you find yourself needing to zoom beyond 3X.
Adjusting "White Balance" may improve color rendition
Most digital cameras are pretty smart with auto-focus and auto-exposure but aren't as good at judging the type of light present when its flash is not being used. The camera can internally adjust for different types of light source by use of White Balance controls. Sunlight, shade, florescent, incandescent, etc. To get to and use the white balance controls you may need to set the camera to manual first.
Not a camera setting, but Gamma is a good fix for dark photos
Many photo and graphics editors have a Gamma adjustment control and it has the effect of lightening or darkening a photo usually better than brightness and contrast adjustments. The next time you end up with a perfectly good photo that is only just a little too dark try it. That's what is programmed into the iDrive Optimizing Tools for perform brightening.