Buying a puppy is a very risky business these days. Under no circumstances do you EVER buy from a pet shop. Not even because you feel sorry for the animals. Every breeder should have a breed standard on hand and they should be able to point out major, minor and disqualifying faults. They should be aware of the major breed problems.
Every breed has some.
These are problems that occur in the breed with more or less frequency that all decent breeders try to eradicate. The breeder should know their own pedigrees well. They should know the faults their own lines may carry. All lines have some faults. They should be willing to put in writing what they will do if your puppy develops any of these problems.