Great questions, but I always like to add, "A great breeder who cares ask YOU the questions."
I too agree with Rodney. I will also add some more areas I would advise looking into before buying.
I know you specifically asked for questions to ask a breeder before buying but instead I am offering some comments from which you or whomever can form your own questions, if you agree (hope they will help in some way):
I feel that we should not buy from breeders or hobbyists who care more about the dollar bill than they do about what is best for their birds. I personally would not buy from or support or refer a person to a breeder who clips or promotes clipping or advises the buyer to clip the bird themselves - period. I feel a great breeder should instead be trying to get the buyer to fully understand all the benefits of full flight and insist that the buyer first be able to prove to the seller that he/she has already built or purchased a full flight aviary or has prepared a room or section of their home for safe full flight.
I would not buy from a breeder/hobbyist who is in a rush to sell their birds but instead I would want to make sure the birds are fully weaned and have mastered flight, meaning they can bank, fly up, down, hover, fly up off the floor and not into things etc. … Sure there will be some additional learning of the new floor plan in their new home but with the skills mentioned already learned, that should go quickly & safely.
I would want my birds to be eating fresh veggies, fruits, nuts, seeds, other healthy & natural foods, (etc.) and maybe some good quality pellets before I took possession of them.
I would want my baby/bird to be unafraid of every little thing placed inside his/her cage, therefore I would want them introduced to various kinds of toys and natural foraging items while they are being weaned.
I would prefer they be socialized around other birds, either of their own species, other species or both.
I would not want a bird from a breeder who smoked or allowed smoking in their home - anywhere.
I would not want a bird from a breeder who thought it was fine to let birds play or come in contact with the breeder's or new owner's dogs or cats or reptiles, etc.
I would want my baby to come from healthy parents, both mentally & physically. I would want to know that the breeder/hobbyist allows all his/her birds (breeders and other birds) direct natural sunshine (not thru window glass). If this cannot be done yr. round then I would want the breeder to show me their F.S. Lighting system and give me the specs of the tubes they are using etc. … Proper lighting (indoor & outdoor) for the parents (& the baby) is essential IMO & IME in getting a healthy and good quality youngster.
I would also talk to the breeder/hobbyist as much as they will allow (in writing or phone calls or both) on bird related topics especially, in order to try and feel out their knowledge, character & personality as much as possible. I think this helps in getting a good feeling for whether or not the breeder/hobbyist really takes care of their birds & whether or not their birds are probably of good quality or not.
I would also want to see the breeder/hobbyist ask the potential new keeper tons of questions to determine whether or not his/her babies are going to a great home, where they will be all they were meant to be and enjoy all they were meant to have - best possible in captivity anyway.
I would ask the breeder/hobbyist if they are willing to take back the baby should the new keeper not be able to continue giving the bird a wonderful home (providing of course that the baby was kept properly and healthy at the buyer's home). Of course the buyer should not expect a full refund if the seller agrees to this. If not willing to take back, then would the seller/breeder/hobbyist be willing to help the buyer (with all his/her contacts) place the bird in a good home.
Is the breeder/hobbyist willing to respond to general or emergency questions when & if needed, even if that meant waking the breeder/hobbyist up in the middle of the night? IMO, a yes answer to this question will in itself somewhat prove whether a breeder/hobbyist really cares about his/her babies, even after they leave their place of birth.
All the other respondents in this thread came up with good questions also and good advice. However, I personally don't think every single question mentioned in this thread needs to be asked nor do I think all birds need to be vaccinated against whatever nor even vet checked before a sale, in order for a buyer to feel that he/she is getting a healthy bird free from disease. No reputable breeder who loves their birds and all birds in general would ever sell a bird to someone if it had any chance of being sick. Good breeders and hobbyists know their birds and have a reputation to uphold. If they had to vet check every bird for whatever and do all the vaccines that anyone may want, their sale prices would have to be very high and they could not then afford to stay in the hobby/field. However, if a customer is willing to pay additionally for a vet check for whatever and any desired vaccines (etc.), then of course the seller/breeder/hobbyist should comply.
IMO, one of our jobs or responsibilities as potential customers & bird lovers is to search out and find the good breeders and pass the word along, either openly or privately. Also our duty should be to shun the bad breeders and pass that info. along too (why they are not good) - openly, if allowed or privately.
All the above is JMO.