Buying chickens is always exciting; but we all want healthy ones. I have included a list of things to look for in chickens before you buy them 
First, decide wether you want pet or show quality. If you choose show quality, find out what the standard of perfection is for the breed. Then go searching for a breeder or someone who has a bird. When you go to see the bird, make sure to look for obvious show disqualifications (missing toes, etc.), lice/mites, and any other signs of illness. Once the bird has been thoroughly checked over, it can be brought home and quarentined.
For pet quality, simply check the birds for illness. Pet quality aren't meant to be shown, and usually have a few disqualifications. However, they make good pets. For a pet quality, you do want to check the temperment as well. Sometimes, imperfect birds can be bred to make show quality offspring (if the chicken lost a toe to an injury, for example.) In that case, make sure that the disqualification is from an injury and not genetic.
Even if the bird is healthy, it still has to be quarentined. Some diseases can take awhile to show up; So prior to bringing the bird home, you should have a pen set up for the new birds.
When buying roosters, you can often find out how he'll behave by how his father and grandfather behaved. If you can see the father, that's great. However, you can't always do this, as some birds come from hatcheries or other sources.
Always look for these signs of illness:
Pale comb and wattles,
Discharge from nostrils/eyes
Mites or Lice (little bugs crawling on the feathers/skin)
Wheezing
Hunched up appearance
Limping
Sneezing/coughing
Lumps inside of beak
Prominent keel bone
A healthy bird:
Notice the nice red comb and wattles, the shiny feathers, good posture. The eyes are clear and there is no discharge. The bird is not overly thin, and keel bone is not prominent. She's well feathered as well.
An unhealthy bird:
Notice the hunched up appearance (in this case it was Cocci). In this case, it was a younger bird. If it was an older bird, the comb would be a little pale if it was the same color as this bird's.

First, decide wether you want pet or show quality. If you choose show quality, find out what the standard of perfection is for the breed. Then go searching for a breeder or someone who has a bird. When you go to see the bird, make sure to look for obvious show disqualifications (missing toes, etc.), lice/mites, and any other signs of illness. Once the bird has been thoroughly checked over, it can be brought home and quarentined.
For pet quality, simply check the birds for illness. Pet quality aren't meant to be shown, and usually have a few disqualifications. However, they make good pets. For a pet quality, you do want to check the temperment as well. Sometimes, imperfect birds can be bred to make show quality offspring (if the chicken lost a toe to an injury, for example.) In that case, make sure that the disqualification is from an injury and not genetic.
Even if the bird is healthy, it still has to be quarentined. Some diseases can take awhile to show up; So prior to bringing the bird home, you should have a pen set up for the new birds.
When buying roosters, you can often find out how he'll behave by how his father and grandfather behaved. If you can see the father, that's great. However, you can't always do this, as some birds come from hatcheries or other sources.
Always look for these signs of illness:
Pale comb and wattles,
Discharge from nostrils/eyes
Mites or Lice (little bugs crawling on the feathers/skin)
Wheezing
Hunched up appearance
Limping
Sneezing/coughing
Lumps inside of beak
Prominent keel bone
A healthy bird:
Notice the nice red comb and wattles, the shiny feathers, good posture. The eyes are clear and there is no discharge. The bird is not overly thin, and keel bone is not prominent. She's well feathered as well.
An unhealthy bird:
Notice the hunched up appearance (in this case it was Cocci). In this case, it was a younger bird. If it was an older bird, the comb would be a little pale if it was the same color as this bird's.