As a buyer, I think it's pretty much the same as normal, just of course making sure it's clear in the advert that the chin will be held for show, and what the terms are if something should happen. I think if it's at all possible if an animal becomes sick or is injured the buyer should still be given the option to have that particular chinchilla once they're recovered, if they still wish - chinchillas.com for instance I think requires the selection of a replacement, but I know if I buy an animal, I do want that one (I'm with danilinn and her decision on her girl with the torn ear), not a different one no matter how similar-looking! A deposit is probably a good idea, that way you have something concrete that suggests they will follow through and buy the chin, and they also have the assurance that they do partly own the animal.
The one thing that always worries me, although I'm sure most chin breeders wouldn't do this (have known cat and dog breeders do it though), is if they're wanting to show the animal, will they end up changing their mind? What happens if the animal does better than expected? So just a bit of reassurance that they're definitely going to be able to take their chosen chin home would help, from a buyers perspective. Even if the breeder only asks if they can keep the animal after all, it can be pretty difficult for a prospective pet owner to be placed in that position of either having to say yes and lose their pet, or say no and take the animal from the now-reluctant seller, so it's obviously best to be sure.