Baby hamsters for sales should be at least 4 weeks old and weaned off there mother before being sold to the public. always check there age before buying your new baby hamster .

You should check the following :

There eyes ?

There body for bites and wounds?

The level of cleaniness ?

Is your hamster playing with other hamsters or is it sitting alone in a corner ?

Does the hamster look fit and well ?

Check fur is all intack and none is missing !

Always ask for a hold of your new hamster before buying to get a closer inspection and to find out the nature of it eg does it like being handle etc.

Dont forget to buy the same food as its been feed in pet shop as change of food at any age can cause stomach upset in any hamster .