I can understand being personally wounded to have your beautiful puppies deemed "unwanted" by someone you trusted to love them for their whole life! It does sound like a lot of dogs to take back all at once, and I'm sorry for your string of bad luck. Regardless, I thank the world that there are responsible and sympathetic breeders like you who will not turn your puppies away. I hope other experienced breeders here can share their insights on how to ensure successful homes, because I'm sure there are a lot of subtle things that tried-and-true methods that I never would have thought about.
Honestly, in the end it is really a crap shoot…. because sometimes people are really good at telling you what you want to hear... sometimes you go with what you think is the right home, but it turns out it is not.... I don't think there are really tried-and-tru methods unless with a prior Basenji owner (my favorite kind of home)... new owners that have not had Basenjis or maybe no dogs at all, you just never know. I don't look for show homes, if I get someone interested in showing that is super.. but it is not a requirement for me. The home is the most important.