
A verbal agreement IS legally binding... the problem is proving it.
As far as sales contracts go - they have SOME worth on the endorsements front, ie, IF you explain and the buyer says they have understood and signs to that effect, that you will not remove endorsements on a pups paperwork without them fulfilling some conditions (or indeed that you won't remove them at all, ever) then that is fine and they haven't a leg to stand on say in a small claims court, to MAKE you remove the endorsements.
Doesn't mean they can't breed the dog - just that they cannot register the offspring with the KC.
On the return of pups issue though - thats a different matter. Once you enter into a contract of sale with someone and you exchange money for goods, the buyer owns the goods and can do what they like with them - in those circumstances, a document insisting that the goods be returned rather than sold on is not (unfortunately in this case) deemed 'reasonable'.
Wouldn't stop me putting that in though - and some rescue centres I believe HAVE forced people to return a dog according to the contract signed (but they dont 'sell' dogs, you 'adopt' one for a fixed, non-negotiable 'donation' (ie, you buy it for a price!) but its very dodgy ground indeed as far as what the outcome would be if you tried to enforce it.