On Craigslist it is a bit difficult. You can use an image instead of text... but then it is not really searchable anymore. You want to put in text that people are going to search for. Since Craigslist is a free ad placement site, there's really no way of stopping someone from throwing up a scam ad in the rental section. I would suggest just putting a disclaimer in your ads saying to the effect that these properties are NOT for rent and if you see these in the rental section please flag appropriately. If people end up getting themselves scammed on Craigslist they really didn't do their research into the property. You can't really be held accountable for someone not doing their research. Just make sure you educate your sellers that this may happen. If the property IS a rental property, give the ads some sort of rememberable quality like a logo or HTML template and again have a disclaimer that if the property is seen in another ad to flag it.