My airsoft gun wont shoot. WHY?

i bought a used gun off of Ebay, and it wont shoot. It is a SR4A1 by SRC and when i pull the trigger, nothing happens. I charged the battery for about 4 hours, and i wouldn't get warm. I still put it in my gun and expecting a noise or a couple of shots, but still i got nothing. i contacted the seller and he is saying the motor is to tight, but i don't think that is the case. Is it the battery or the motor, or something else.