Chad said that generally they will offer a "safe check" like a cashiers checks. Then they accidentally overpay (by a long shot) and ask you to refund the difference OR they actually come by and pay with a "safe" check and the check always turns out to be a worthless check printed from a laser printer that will take several days possibly to show up. They are trying to come across as an honest and credible person. Either way, they are most likely out of state or out of the country which means very few if any law enforcement agencies will or can follow up and if so, few counties/ cities will pay for extradition of a criminal back here for an offence like this. Like id/ credit card theft. The bank or credit card companies almost always reimburse you so in the eyes of the written law they become the victims, not you. Because it is not worth their time to spend money on helping prosecute these thieves (such as employee time assisting police) they typically don't worry about charges that are $10,00 and even more unless the criminal and the bank are local. Law Enforcement has few choices but to drop the charges at the banks request and these criminals know it.
This email could be a "bot" and just send these replies out to all the ads in the area it's scanning. If they get a reply and it's on something they can make good money on, then they follow up on it to see how far you will go. Could be a lot of different scams.