When it comes to LH guns you need to find out if its store owned or if it's on consignment. Many LH gun owners, as you've demonstrated "over value" the guns because there are "so few" out there or have emotional attachment and they sit. A LGS owner knows they don't move quick and often pennies on the dollar, offer to only do consignment, or outright pass on the purchase/trade. I see it daily. Conversely, there are certain LH guns that move extremely fast and do command a premium, think LH Citori, Silver Pigeon, Rem 700BDL in a common caliber, etc. They're typically current production guns in the $1500-4K range new, but not produced often as a LH. However when they're DG Calibers, obscure calibers/builds, or bespoke guns, the odds are more in your favor. Look for dust on them or a consignee that's been there forever.
I'd bet
@rookhawk is probably pretty close on how many used LH guns you could get by lowballing every used LH rifle on GB and GI offering 20-40% of asking price. My dad is RH shooter who went blind in his right eye and had to learn to shoot LH. I've bought him several LH guns over the years employing this same lowball tactic, especially when I see dust on them.