H&K do not make airsoft guns.
H&K designed the layout of the outer body, and will have licensed any trade marks on the gun.
Sorry I didnt notice it was 2nd hand in the OP. I'm inclined to say you were scammed, but without knowing what brand it is, its hard to say if the price was bad, ask the guys that sold it to you, if they didnt know what brand it is, there is NO WAY they could have chosen that price in good faith.