I vend expos and sell online, and in both cases I have a 'live and outwardly healthy at the time you get the animal' policy. I give 24 hours on shipped animals to confirm the sex. That's all in writing. I've gone over and above this on more than one occasion, and usually the buyer is clearly the one at fault, but I think it good to have a firm policy in place.
I think the extended (30 day) health guarantees are a good business decision for those sellers who want above all else to maximize number of sales. The number of buyers who will make a claim on an animal they killed or isn't even sick or dead might be outweighed by the increased sales volume such a liberal guarantee will encourage, much like retail businesses with a ridiculously buyer-focused return policy. I personally don't try to maximize sales -- quite the opposite, in fact -- but some vendors certainly do.