Hey there, welcome to HC!
Hm. To be honest, it really depends on your area. I myself work in a pet shop, where all of my hams have come from, and they have always been perfectly healthy, good natured pets with beautiful colours and great personalities. But, some pet shops do have bad stock (*cough* Pets at Home *cough*) and as stated above, stress can cause many pet shop hams to contract wet tail and other ailments.
If you know good breeders, I would stick with them. Bare in mind though that some breeders can be just as bad as some pet shops, so it really depends on your area and what you can learn from your source. It's important to always ask questions.
And the normal age for selling on is 5 - 6 weeks in age.