I've added some picture library links to the site. As you can see from those and the above example, selling pictures online is all to do with how your images look as thumbnails. When you click on a thumbnail the example image is not much bigger and certainly smaller than the pictures I post here and on flickr.
Its from these that clients make a decision as to whether to buy that image or not.
So as far as the picture buyers are concerned, the camera used is irrelevant. The exif data is stripped out anyway in most cases when they download the images, so they will never get to see what camera the photographer used, or details like aperture, shutter speed, focal length etc.
Where all of that does become important is when you submit images to the library itself. Since the image has to get on the site to sell, thats when the quality / size issues etc. become important and thats why the quality of the camera and the lenses used can make a difference as to whether a file gets accepted or rejected.