There are several aspects that we should consider when buying a hamster:
 
1. A healthy hamster
  -should not be too thin
  -have bright and clear eyes
  -have round body
  -have dry nose (shouldn't have snivel)
  -should be active
 
 2. The hamster shouldn't be too young (at least three weeks old) or too old (not more than three
 months old). As young hamsters are easier to tame. However a hamster under three weeks old
 is not able to feed itself suitably.
 
 3. Buy hamsters in the afternoon. As hamsters usually sleep in the morning and you can't see how
 active they are.
 
4. Male / Female?
If you are going to keep one hamster only, there won't be much difference for keeping a male or
female hamster. However, if you don't want the hamsters to have babies, you should keep hamsters
with the same gender.
Here is a picture showing the difference between a male and female hamster:
 
5. Beside different appearances, different species of hamsters also have different characteristics.
Here are a brief introduction to some common species:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Syrian Hamster / Golden Hamster

Scientific name
Mesocrietus aurantus
Size
male: 12 -18 cm / female: 13 -19 cm
Weight
male: 80 -130 g / female: 90 -150 g
Pregnancy
about 16 days
Number of babies
 3 - 17
Life span
 2 to 2 and half year
Characteristics
 friendly, prefer to live alone

 

 

 

 

 

 

Russian Hamster

Scientific name
Phodopus Sungorus
Size
male: 6.5 -11 cm / female: 6.5 - 10.5 cm
Weight
male: 35-43 g / female: 32-37 g
Pregnancy
about 3 weeks
Number of babies
 1 - 9
Life span
 1 and half to 2 years
Characteristics
 easy to get along with, prefer to live alone

 

 

 

 

 

*Little tip: This is the kind of hamsters I used to have. I strongly recommand you to keep one russian hamster in a cage only. I didn't know they prefer to live alone when I bought them, and end up having the two of them fight all days. If you decide to breed them, you should see if the two hamsters get along with each other. And should also separate them with different cages.

 

 

 

 

Campbell Hamster

Scientific name
Phodopus Campelli  
Size
male: 7 - 11.5 cm / female: 7 - 11cm
Weight
male: 35 - 45 g / female: 32-40 g
Pregnancy
about 3 weeks
Number of babies
1 - 9
Life span
1 and half to 2 years
Characteristics
 more difficult to get along with, sensitive and often being nervous

 

 

 

 

 

 

 

Roboroskii Hamster / Desert Hamster

Scientific name
Phodopus Roboroskii  
Size
5 - 7 cm
Weight
15 - 40 g
Pregnancy
about 3 weeks
Number of babies
1 - 9
Life span
3 to 3 and half years
Characteristics
nervous, timid, sociable (can be kept with other desert hamsters)