To my knowledge, there isnt' alot of morphs that only show up as the animal ages. But the odds on getting one, even from a clutch born of parents who show the desired 'trait' are not always 100%, especially if only one parent has the visible trait. Most animals that carry the recessive mutation, don't show that morph at all, as the normal gene is dominant. However breeding hets for this mutation have a greater chance of producing the final desired trait in one or more of the offspring.
Most breeders sell normal looking 'hets' as possible hets. Though it is possible to have 100% hets for a certain trait, usually because the possible genetic combinations always produce either a het or a full visible examine of that morph.
The high price of ball python morphs, from what people say in that forum is a combination of relative rarety, as not all morphs developed have wild caught examples, small clutches with mostly single clutches per season and high demand. If people buy $23000 dollar snakes regularly, breeders will keep the prices high. I have seen prices as m uch as $150 000 for ball python morphs, which I consider insane. There isn't an animal I find beautiful enough to want to pay that much money for. Though an all white blacked eyed snake may be cool looking, I still like the natural pattern better.
Rainbow boas, other than subspecies (Brazilian, Peruian, Columbian etc) don't seem to have much natural morphs that I have heard of so far, except for rumors of an albino Brazilian on this forum. A ruby BRB or patternless, may fetch a higher price than a regular BRB, but if it would reach the extreme hights of ball python morphs, time will tell.
For breeder responsibility, as original question asked, it would all depend on how the snake was marketed. If it was explained at purchase, that the snake may change color once it was a couple years old, and it never did, that was the risk the buyer was aware of, at purchase. If it was marketed as a guarenteed to become a ruby snake, then some sort of recompense could be expected. I certainly can understand a seller being reluctant to give a full refund, maybe a free snake with the same chances, but the seller m ay be concerned that the 4 year old snake was even the same one he originaly sold.