Certain guns will sell faster and easier than others. Thousands of used guns are selling on Gunbroker, ArmsList, firearm forums, used at pawn and LGS, etc. each and every day. Even more NIB guns sell.
Have you ever simply thought that their isn't much of a market or interest for whatever gun you're selling, you're trying to sell it on the wrong platform, or that others don't think it's worth what you think it's worth?
I also often hear some people mention the MSRP, that they paid X amount NIB retail, and are selling it for $50-$100 less than what they paid. What I've often witnessed is that there are rebates or online vendors who are selling the same gun NIB for only $50+/- more than these many are trying to sell off their used guns for. Buyers will rather pay the extra $50 and buy the gun new from the manufacturer; I know I would. Plus, many warranties only apply to the original purchaser. It does not matter how much, for example, the OP originally paid for his used gun. What matters is the current market and what others are selling the item for both new and used.
OP, being that most potential buyers will be right handed, I don't really think your lefthanded holster is sweetening the deal.