You will loose money on a direct sale or trade to an FFL holder about 99.9% of the time.
A FFL will have to mark up the gun to make profit, this means he's going to offer you as little as you will accept in order to make more profit, that profit is what you are loosing.
If money isn't tight and you're willing to loose up to $100+/-, sure trade/sell it to a dealer.
But I will say this, you have your two carry guns, you haven't specified that there is "X" gun at the shop calling your name so you have the time to post it for sale on the boars, Sigs usually sell quick and hold their value so you won't loose much ( in the great scheme of things) on it at all.
You may not have to ship the gun FROM an FFL as long as it is both legal in your state to ship a gun Fed-Ex (about $30.00 for overnight withough insurnace) and that the receiving FFL holder will receive guns from a non FFL holder (that info should go in your sales ad), if those two things are OK, you don't need to go through a dealer unless you want to pay less $$ on shipping. A FLL holder can ship a handgun via US Mail (3 day) for about $20.00.
if you don't want the hassle of shipping the gun, just put it on consignment after checking around for rates. It doesn't seem like you're in a hurry to axe the Sig, so waiting a while won't hurt you and it will give you more time to shop around. Just be advised that if you put the gun on consignment, and it does not sell and you want to get it back from the dealer, you may have to do/pay a back ground check to get your own gun back.