RiceAWay
Many of those who are on the classifieds (and other sites) are bored and lonely cell phone addicts; classified ads are a big opportunity to interact with a non-AI persona. They need an excuse to remain glued to their "sacred screen" like the ancient Narcissus who peered at a reflection of himself all day. Sending messages to advertisers and receiving responses is a way for many to "feel busy" like they are doing something with their lives and to extend those precious seconds of extra screen time. One way to weed out these people is to ask they leave a voice phone number to call in their response (or in your ad in the first place requesting it). If they won't leave a voice message to call for arrangements, particulars etc., then don't deal with or respond to them. They just want to use you for your energy.