I had to put on some strong reading glasses and back away from the screen before it became clear. No, the parents aren't selling it with crime scene tape on it. That was taken before it has been released. The photo was taken in a parking lot, either at the sheriff's dept or crime lab type place. Here's another photo which shows both Maria's car and Cesear's truck.
Maria wouldn't have been driving around with that sign up in the back. I don't know if she was selling it, but what a handy thing to put up in the car. It makes it seem more reasonable if someone sees it sitting in a parking lot for a long time. They think someone's parked it so they can sell it. Plus the sign looks hurriedly penned. There appears to be more writing below the FOR SALE lettering, but I can't make it out with the reflection and how light it is. Any ideas what the blue and white thing is? It almost looks like a baby blanket or a cozy for a car seat. She thought she was having a boy. But why would it be in the rear window blocking her view?