Emily's comment elsewhere led me to her site and this clever piece of marketing.
Tapping into already interested prospects like this is at the heart of all our marketing efforts. It seems to me that there must be a way that the web can automate this sort of reverse viral approach. It's similar to the Amazon recommendation ethos, but a little more subtle I think.
Regardless of that, we can all learn from the way this marketer has made his product remarkable: firstly by separating it from the mass of other used cars on the lot or featured in pages of ads and then by harvesting the power of personal recommendation.
As I have written before, casting where the fish are doesn't have to mean casting where everyone else does.