There are many people in the world who put a male and a female animal together, and call themselves "breeders".  It takes much more than that to produce healthy, happy animals.  Some questions you should have answers to before adopting a rat or four(!) from a person:  

Where do these rats come from?
It happens quite often that a person brings a female home from a petstore, and she has a litter a week later.  Nothing is known about the background of the animals, but they can still be wonderful and loving pets.  If the person says "I bred them" you want to ask more questions, to find out what kind of "breeder" they really are.

Can a pedigree be provided for the rats?
A pedigree is just a family tree for the rats.  It shows who their parents, grandparents, great-grandparents were and who bred them.  Rats from a petstore don't come with a pedigree.  So be cautious if a person says "they don't have one."  

Are there any health problems in this line?
Serious breeders track their lines over many generations, and know what problems may occur.  People who breed petstore rats often will have no idea.  You may be lucky and get rats who are healthy most of their lives, or you may have to start medicating them at 3 months old, and they live a much shorter lifespan than "normal."  

What kind of bedding do you use?
Pine and cedar are cheap beddings, but they contain phenols which irritate the respiratory systems of rats, and have the potential to cause internal organ damage.  Good beddings include Carefresh, walnut shells, aspen, corn cob, cloth, shredded newspaper (with soy ink), or paper based litters like Cell-Sorb or Yesterday's News (Biocatolet in the UK).

What kind of food do you feed your rats?  
Rats should not be fed hamster, guinea pig, or bunny food.  Rats need a food formulated especially for their nutritional requirements.  Seed mixes are cheap, but they are not very nutritionally balanced, nor do rats eat all of them.  A lot is wasted.  Lab blocks or homemade mixes can be cheaper, and provide better nutrition.  You may want to feed your new rats the same diet as their breeder did, to help them acclimate to living in your house.  

How many litters do you have a year?
Many reputable breeders only have 3-8 litters a year.  Beware if a person gives you a very high number like 20!  They may be breeding litter after litter to supply to petshops, or even to supply snakefood!  

Finally, don't be surprised if the breeder asks YOU questions, or requires you to sign an adoption contract.  They work hard to produce beautiful and loving rats, they want them to go to the best homes possible.

More information can be found in the Reference Desk, as well as Rat Care - a thread titled "Personal Stories About Breeding" shows what horrible things can happen when petstore rats are bred.