Dunkel, you cant take responsibility for what happens after the buyer takes delivery. You are spot on with producing a quality product. i agonize over which nuc I am going to sell to whom, and then worry that something bad will happen, the queen will poop out etc..
If they let the nuc swarm, that is up to them. If the queen does poop out, then I will replace her. Honesty is getting harder to come by these days, and many have decided that if they are going to be lied to and cheated then they might as well call it even and do the same. That attitude doesnt cut it here. We may pay the price for honesty, when it seems so few are honest these days. To me, it doesnt matter how dishonest others are. I take some pride in others coming to me because I am honest, but what really matters to me, is how I feel about myself.
Do the best you can for them, and let them learn from their own mistakes.