Why do people do this?

I've had several parts wanted ads running recently. In the last month or so I've had no less than five members respond that they have what I am looking for and after I express my interest and ask for pics, price, etc, I never hear from them again. I don't get it. Why tell someone you have the part and then never follow through on it? I get all enthused thinking I may have finally found what I need then I get no further correspondence from them. This isn't happening with new members who just signed on that might indicate a scam either. These are people who have been members here for years.