Well... A guarantee period is really a direct respresentation of how confident a breeder/dealer is in the quality of their animals. Minor husbandry mistakes won't take an animal out for a week or two generally so a longer period for a guarantee simplt means a more confident dealer... only dealers who can afford to be confident and stay in business long as those which warrant it. Mistakes happen even to the best individuals, the true measure of a dealer isn't how they take care of customers when everything is going right, it's how they react to problems. Personally I feel much more comfortable buying from someone who's been the victim of a third party mishap and took the loss and protected their customer than someone unproven.
As a bare minimum live arrival/healthy arrival is inherently guaranteed for non-wholesale situations or when not discussed in advance of the sale. The customer paid for a live animal and probably paid for a healthy animal so anything less than this is simply misrepresentation of a product. Anything above and beyond that is entirely the result of the dealer's confidence in their animals.
Always remember that, as a dealer, the buyer is a supplicant, they approached YOU about an exchange (of either money or animals or goods/services), by making your terms of service clear from the outset you protect yourself for potentially negatove situations. There needs to be a cutoff point where a dealer is no longer responsible for an animal and can't be forced into culpability ("Remember, that snake I bought two years ago? It died, you owe me a new one." is a dealer's worst nightmare). Plus if they're YOUR terms, YOU can always choose to step outside of them. If for instance you offer a one week guarantee, a customer purchases an animal and verifies that it arrived alive and healthy, eight days go by and the animal succumbs to an internal deformity that neither you or your customer could have forseen- you can choose to issue a refund even though your guarantee technically expired in the interests of customer service and because it's really the right thing to do or you can choose to write them off (and never see a repeat customer again), the choice is yours.
A terms of sale page is possibly the second most valuable characteristic a dealer can have... A good solid well earned reputation is first, but every action a dealer takes, every sale they make which never requires the guarantee to even be considered, animal they produce that ends up with a happy customer and person they help get started all work towards establishing that reputation.
"Genes, Like Leibnitz's monads, have no windows; the higher properties of life are emergent... And once assembled, organisms have no windows." - Edward Wilson, Sociobiology