Unlike the rat fancy, which is more pet orientated, the UK mouse fancy does not use contracts. Private breeders may do, but to be honest since everyone knows they're not legally binding it doesn't matter if someone signs it or not - it doesn't stop a potential bad owner because they are not bound by it, and tbh it can be hard if not impossible to know what happens to an animal once it leaves your care if the new owner refuses to keep in touch. Since the UK mouse fancy is a case of all the regular show-goers know each other, that's why there isn't a problem swapping or giving mice to each other. As has been said it's ironically members of the public who want pets that tend to be the problem owners. I've had some lovely people have pet mice from me but I've also had a couple who seemed fine, but turned out to be very poor owners despite having other mice. In one case a friend of mine went and collected the mice I bred and was shocked at the state they had been allowed to get into (they're fine now).