the root of the problem is that in pet shops syrian hamsters are being kept in groups while they are by nature solitary animals. Thus they are under constant stress and that makes them aggressive.
Another facet of that prob is that syrian hamsters have no bite-inhibition reflex like most other mammals. It's simply not necessary for them as in nature they have enough space to run away. In a pet shop or the buyer's cage they can't escape an attack and therefore will almost inevitably be killed by the stronger or more aggressive specimen. This goes particularly for those genetic lines that most resemble the wild form (brown and white, short fur). Long-haired breeds are less aggressive (propably because they trip over their own fur if they charge...). Females are always more aggressive than males.
If hamsters were kept single in pet shops they'd all be quite friedly little chaps.
If you get such a traumatized animal, the best strategy is to leave it alone until it has calmed down and feels safe in the new environment (usually 2-3 weeks). Then let the hamster make the first step - if it gets bored it will seek contact with you all by itself. Lure it to your hand with a yummy mealworm or an almond so that it learns to connect you with a positive experience. This way you'll tame the worst fighter within 6 weeks
(In really bad cases, try mozzarella cheese or a drop of plain yoghurt - no rodent can withstand that. Only tiny bits, though, else the hamster will get indigestion! Please never feed chocolate, cookies or other sweets - almost all old hamsters and many young ones are diabetics)