You hear the term all the time - "Reputable Breeder". What does it mean though? With dogs (most people are familiar with dog breeding practices on some level) a reputable breeder is someone who has had all recommended and required testing (genetic, x-rays, blood, etc.) done on their breeding animals to be certain they are not passing on undesirable traits. They love the breed they're raising and their goal is to better the breed. They take exceptional care of their animals, and are selective about who gets their animals. They are knowledgeable of the breed, and of care for them in general.
While the love of the breed and knowledge of care side of the equation is true of reputable rabbit breeders, the rest is a little different.
There are no genetic tests for rabbits
While it would be great to be able to test rabbits for genetic malformations like malocclusion, heart defects, and other maladies, it's simply not available. A reputable breeder will cull (remove from the breeding program) any animals found to have these traits, as well as the parents of the rabbit. That may seem unnecessary, but the only hint that a rabbit is carrying the gene for a congenital defect is a kit being born with it. This means (for many defects) that even rabbits born from the pairing that appear healthy may carry the gene and pass it on to future progeny. The only way to stop it for sure is to stop breeding those rabbits.
They answer your questions
A reputable breeder will answer any question you have, to the best of their ability. While ALL breeders deeply appreciate you googling your own answers to more mundane and easily answered questions, your breeder should be willing to answer questions you're unsure of. They should be knowledgeable of both the breed(s) and the individual rabbits they have. They should easily be able to tell you what they feed their rabbits and why. They can give you tips on the health of rabbits and also what to look for.
They do not pressure you
A breeder should not be pressuring you to buy a rabbit. If you're getting used-car-salesman vibes from your breeder, you should probably reconsider your purchase. It's incredibly unlikely a breeder is going to deliver the rabbit to your house and they also shouldn't require a deposit within 5 minutes of speaking to you. If you asked what the required deposit is they should answer, but they shouldn't be telling you "there are other people who want this rabbit, pay the deposit now or you're out". Honestly? In life in general - don't buy anything from anyone who acts like that.
They are available for help
Rabbit savvy vets are not plentiful in the US (or many other countries). Because of this, simple health care questions are often answered by google or a breeder. While they are not a replacement for a Vet in an emergency situation, they may well be your "first responder" so to speak. If I had a nickel for every panicked phone call or message I've responded to I'd have - well, thankfully only a few bucks. But the point is, your breeder will hopefully be able to walk you through most things.
Their rabbits are healthy / Closed Rabbitry
Refer to the post Is This Rabbit Healthy for tips on recognizing a sick or improperly cared for rabbit. A breeder with a Closed Rabbitry, who refuses to allow you to see their rabbitry in person, is not necessarily hiding anything (when I first got into rabbits I saw some rather horrific situations that breeders had no problem letting me into), rather, they're protecting their rabbits. There are so many illnesses that can be picked up and carried in on clothes, shoes, hair, skin, tires, etc. that it is dangerous to allow random people into your rabbitry. If, like us, your breeder also raises chickens, turkeys, quail, or other animals - it is even more important to keep random people and the things they may be carrying away from the property.
This may sound paranoid, and to a degree it is, but once you get some illnesses it is almost impossible to get rid of them and they could cost you your entire herd/flock. RHDV (Rabbit Hemorrhagic Disease Virus) and AI (Avian Influenza) are two such diseases. There is no cure, no treatment, and no stopping either of these diseases once they enter your herd/flock. The result is devastating. For this reason, many breeders have Closed Rabbitries. It is not an insult to you, nor is it a matter of not trusting you. It is a matter of protecting our animals.
This does not mean that a breeder who will let you in has unhealthy animals. It simply means they are more comfortable with the consequences if they do end up with disease. For us, we have some very expensive birds, and 2 rare breeds of rabbits that were quite difficult for us to find and that we have put a lot of resources and work into. None of these are animals that can be easily replaced and we are determined to protect them every way we can.
Transition Food
A breeder should send you home with, not only the rabbit, but also feed in a sufficient amount for you to switch bunny to the feed they will be getting with you. A rabbit's sensitive GI tract does not appreciate sudden feed changes, and this can make your rabbit very sick. Feed switches should be as gradual as possible, and should be accompanied with hay, particularly for young rabbits. For young giant rabbits a quart zip-lock of the feed the breeder has been feeding is the standard amount you should be given, though some breeders will give more.
This is not an exhaustive list, and some legitimate and reputable breeders would prefer that you just take your rabbit and go. They may not have any desire to get updates, to chat about how they're doing, or to be your first call when something goes wrong. I'm not suggesting that these people are not reputable, but if you are new to rabbits and need a little more hand holding, this breeder may not be a good fit for you. At minimum they should be willing and able to answer questions about their rabbits, like weights, ages, litter size, etc.
Personally, I like to take what I call the open adoption approach. Once approved for a bunny from Where The Gray Bunnies Grow, the contact is up to the buyer. I don't demand updates, but I certainly won't turn them down. I don't demand that anyone message me first with any questions, but I'm okay with it if you do. If you're more of a take the bunny and go person, I'm totally okay with that. Since I approved you to take one of our bunnies home, that means I trust you to care for the bunny without me breathing down your neck.