Rest easy, you didn't get screwed. The price you paid was not the lowest, nor the highest, that a used SW9F will sell for. So, don't worry any more about the price and go shoot the snot out of it.

However, the seller did lie to you, and with a straight face. Feel free to discuss this with him should you see him again. Perhaps you will be so lucky as to point out the error of his statements to you just as he is about mislead another potential buyer...