If you had a type of response in the first place that was on a personal level, and not a commercial level you could always "befriend" them in the sense that your just touching base with them as you think they might be interested in a similar offer.
EX. You ran a ad for car, did the credit report gig, let them know thanks but cars gone. Done. A little time passes, hit them back and say hey I found the same car here, except this time maybe use egay aff link. Could get another $25, or commision off sale. Who knows.
Point is to pose as an average joe who's being friendly and just mailing them back because you felt bad that they didn't get the car.
Just don't start spamming them with unrelated crap, you could increase the value of each lead more than a one time shot.
But anymore than 2 - 3 I think would probably push it.
They have obviously mailed you first responding to an ad, so I wouldn't see much harm as your just trying to help them out in relation to your original ad.
I find that any ad I run with a response that sounds as down home and personal as I can get, brings great results.
I get some practically begging me to hit them back just as soon as I have another.
