You can get the most value selling it yourself. Consignment at a high traffic shop like Four Seasons is #2, followed by trading it in at a shop. Selling to a shop for cash is even worse.
Four Seasons gets an 18% cut on consignment guns. Now, you're down to 82% of the consignment price. If the shop gives you credit for the gun, they are buying it an hoping to sell it, so they'll need an extra edge on top of the 18%. Then, consider that the owner sets the consignment price which may not be realistic for a timely sale, and could very well be higher than what Carl would set as the use gun price if it was in the used case as a shop owned gun. It's easy to see that original number shrinking fairly rapidly.
Or, looking at a hypothetical example:
Assume a gun in "Mint" condition that went for $1000 new is priced at $800 in the used consignment case. (Think 20% is too big a spread? Ask yourself if you take a used one over new for a 10% discount?) Take Carl's 18% cut, and it's down to $656 to the owner. Since outright credit would be expected to be lower than the consignment revenue, even a 10% spread here puts the credit value of a new but pre-owned gun down below 60% of original selling price, and quite possible less than 50% of the MSRP that nobody pays for the mass market/big name guns. As bad as it sounds, if you get 50% of new selling price as credit for a mint condition trade-in, you're really not getting ripped off - just paying the shop for the convenience factor.