Just list it firm and don't even respond to any negotiating. Done. If someone does their research they'll find out what the correct price is, pretty quickly. Only nippleheads are going to pay much more than that."No lowballs!" usually means "I'm asking $50 more than Four Seasons and $150 more than Gunbroker."
Although I get where they're coming from, since every time I put a gun up for sale some comedian PMs me saying "I don't like the gun and it's not worth anything to me, but I'll give you $100 because I'm a Nice Guy if you can't sell it for $450."