I live in Europe in a country where most of the ferrets were imported about 15 years ago and most of the current population comes from only small number of animals. I know about cases when a ferret had the same male as father, grandfather and great-grand-father.
Over the years, a lot of people tried to fix the situation by importing animals from other countries but there are still many animals which die young (2-6 years) because of various health issues.
The longevity of ferrets is most affected by genetic predisposition. Yes, care and diet will affect the longevity, but in-breeding will affect it (shorten it) much more.
To ensure the longevity of your ferret, you should first check its ancestors. How healthy are they? Do they have kidney problems? Are you sure the parents or grandparents were not related to each other?
If the vendor cannot tell you about the ancestors (or show them to you and let you pet them), don't even consider buying a ferret there.
Also note that the character of a ferret can be affected a lot during the first 3 months of its life, which is the time when the little ferret is still with the breeder. Getting a ferret from a big farm usually means getting a ferret that has not been pet at all and it's a wild animal.
I would recommend not to buy a ferret from a breeder that has more than 3 litters a year. He/she just cannot pay enough attention to every ferret and the risk of a wild ferret is too big. Don't buy from someone who actually has ferret breeding as a living.
So the answer to
how can I avoid purchasing...
Invest the time to learn about the breeder and the parents of your ferrets.