Because most people take pictures with a phone that has oily fingerprints on the lens and you can't tell how bad they look on a small screen. That and I'm not a professional photographer.
Personally I don't want to deal with someone asking for a bunch of pictures anyway, if your interested enough in something I'm selling you'll come and look at it. If your the kind of person who needs a lot of pictures to decide if you want something odds are it will be sold before you make up your mind.