I'd ask to see their facilities before you purchase any rats from them. I have to agree with A.ngelF.eatehrs on this one, the whole only males thing sounds awful shady.
A good breeder should have pedigrees, vet records, and should be registered with the major rat organizations. Oh, and most breeders only sell babies (6 weeks is normal) the only adults they have should be the breeders or personal pets.
I remember reading on another forum a while back that a man set up a breeding operation in his home town, he bashed PetSmart/PetCo saying their rats were unhealthy and inbreed (all true) and that his rats were breed for health and temperament. Long story short he would go to the neighboring town's pet store and buy litters of feeder rats for something like $5 each and then resell them himself for $40. The horrors of BYB.
I'm going to be adopting two new girls from this breeder when she plans her next litter, you can check out her site to see if your breeder is up to snuff:
My Breeder's Link
In their FAQ section #15 they say what to look for in a breeder.
I'm not trying to be mean or tear your dreams down by any accounts, but I know there are a lot of people who pass themselves off as respectable breeders when all they care about is making a quick buck. Make sure you're getting the healthy, well bred rats you are paying for.