I think I'm going to get my wife a stamp that says some thing like that. I'll probably note that if it was sold, it was stolen, but not put any personal details on it. It's not like I could really do anything about it anyhow, and people would be mad if I told them that when they reported it.
I was thinking more like "of XYZ town" than anything about you personally. And since most of these are getting sold online, you want the duped buyers to report the sellers to the site where the books were listed. A bunch of one-star reviews for selling LFL books will tank someone out of abebooks or similar pretty fast.
You might even put a stamp on a sheet of paper with a note that it's in every book in the library to discourage the thieves from looking in the first place.
FROM THE LITTLE FREE LIBRARY OF SPRINGFIELD
ENJOY IT AND PASS IT ON
ANYONE WHO SELLS IT IS A THIEF
ONE-STAR REVIEW THEM WITH WHY
ON THE SITE WHERE YOU BOUGHT IT
Thanks for the idea!