Even if you don't purchase anything, Ebay can be a virtual treasure trove of images for your genealogical research. While finding a picture of your ancestor doesn't happen too often, you may find postcards with pictures of where your ancestor used to live, attended church, went to court, etc. And there can be a wide variety of other ephemera for sale on the site. A sampling of items I've purchased recently can be found here.
Even if you aren't able to purchase the item, images can be saved.