With hets, the breeder is guaranteeing that the snake has the gene, they are not guaranteeing that the snake will breed and produce viable eggs. There are so many variables to breeding that I don't think anyone would put their name and reputation on for one specific. And plus say the desert doesn't lay viable clutches the first time being breed how long you you wait. Do you tell the customer you need to try for x amount of seasons before a return could be refunded?
Don't get me wrong it would be amazing for every snake to come with a guarantee that the snake will breed but mother nature doesn't always work like you expect, so I'm pretty sure it will never happen
Tom