I put up a For Sale sign on bare land (no structures) and wrote, "Shown by Appointment Only," along with my telephone number. Most everyone called about the property, but a few people did not and walked out onto the property. As a seller, you cannot insult nor rebuke a potential buyer too strongly. That would ensure they won't be a buyer. I found that when I located someone looking at the property without permission, I would tell them that since the property lines were not clearly marked, I could not guarantee they had seen the correct property. I also warned them that the neighbors might not like somebody crossing onto their property from mine. Folks quickly got the point as to why I wanted to be there. If I had time, I'd give them an impromptu tour, or if not, I'd offer to make an appointment.
I see things posted for sale inside people's fences all the time. If there is no fence or if the vehicle is right at the fence, they probably don't care if you examine it as long as you walk through a gate and don't climb their fence. When I see something parked back from the fence, it's a subtle hint to me that they really want to know when you are looking at it and you should go up to the house or call a posted number. As a buyer or seller, common courtesy can go a long way.
