There is no law against it, however the leading 2 reasons a seller should not be there is that the seller can make the buyer very uncomfortable where they rush through the home instead of taking the time to fall in love, opening cabinets, closets and talking about how their furniture will fit. Second is liability. If you are there, you can say things that hurt your sale more than help. Lastly is any buyers will openly discuss what they like and what they do not. Most sellers get angry and or hurt hearing such feedback as they walk through the home.