If you haven't paid, then sure, you can refuse to complete the sale. Are you morally and legally right. No, you're not. Your post about the seller having the responsibility is just you making up excuses after the fact. They put up a rifle, had its pictures, and gave you all the opportunity in the world to do your research. You didn't, and now you want to back out. What works in your favor is that they will likely not bother to pursue you, because the trouble of suing you is not worth their time. If we were talking about a $500,000 piece of art, sure, it will be different, but this is not that.
So go ahead, be a douche, and refuse to pay. You will get banned from the auction site, but it will likely be the worse that will happen.