Try selling things that aren't guns on FB marketplace or craigslist. Haggling is a way of life on guns or anything else, no use getting offended by it, there will always be lowballs. If someone is close on price, I reply back no, price is firm, or if they're real close, I'll say sure. If someone is crazy low and I doubt we'll find a place we're both happy with, I just don't reply. If your price is truly good as you claim, it will sell to someone at asking, and $600 to ignore some lowballs is a pretty good gig.