As for appearance, keep the color space as RGB which your digital SLR will capture in, unless it is set to "RAW". RAW would be the optimum format to shoot in, as it gives greater freedom for color and lighting adjustments prior to importing it into an image editing program like software. However, for A'Gon purposes, a higher resolution setting with RGB as the color space should be fine.
When I compare the small preview images Audiogon creates compared to the actual uploaded photos in the ads, I can see that Audiogon is running an auto color adjust possibly in Photoshop on the images so they appear richer and deeper in color.
In Photoshop, as long as you haven't reset the white point and black point of your curves or levels adjustments to some crazy color, an "auto" will find the lightest part of the image and set that to specular white and the darkest part to max black. That way dull, hazy washed out images will have deep, rich tones, and vibrant contrast. Not always realistic, but definitely makes for a dynamic looking picture.
Another thing you could do is to run a very mild unsharp mask on the image to sharpen things up a bit. However, if you create enough contrast through color adjustments, any sharpeneing may have the effect of enhancing digital jaggies or JPEG compression which is a bad thing.
Regarding resolution, don't upsample the image you take if your camera is at a low setting. This will cause the image to become softer and will actually enhance some of the compression so image looks poor.
As Aball suggested, shoot it at a higher resolution than you intend to use it at, make all your retouching and color adjustments at that resolution, then crop it and save out a smaller file size for uploading.
We have a lot of photographers on Audiogon, hopefully they can chime in with lighting tips.