As a small breeder reading this thread
It's a really scary thought, you just don't know. who or what anyone has done in the past unless they tell you.
i once phoned a propesctive owners vet (I had their permisson) the vet said
"they couldn't tell me anything about this person as it was private info"

so that really helped NOT.
i just go with my gut feeling and unfortunatly sometimes we all get things wrong, luckly i've never heard of any of my pups having problems with their owners but i know this happens to animals across the UK (& the world) every day and the sooner we can get some kind of data base up and running that we can all use to check bans the better..