You'll only get the answers you are looking for from Skrill. Did you contact the person that sent you the money to explain? Did you get any response? This transaction seems to be the only suspicion in your mind so he may have charged back, or he may have sent you stolen money. How did you find your buyer? Did you ID him?
The man dont reply on skype any more , i talked to skrill and they said that i am under security check , i am very old customer and have traded like 2-3K over there and i dont know why only this time they are having problem.
It sounds highly probable that this transaction is the cause for your account suspension. Give them some time to investigate and ask for an ETA on when it will be resolved. When did this transaction happen?