I think your issue will be a 115hp motor on a boat rated for 175hp. Not that its “bad” in and of itself, but it does eliminate potential buyers. As far as NADA, I think most believe it to be on the high side of true value, its what I believe at any rate. My suggestion would be to consider your cost basis, condition of the boat and, how badly/quickly do you want to sell it…. establish “your price” and list it. There is a 08 Sabre FTD, with 21 hours on the 115hp Merc, listed for $16,850 on BBC and, its been listed there for a long time. Someone looking at a 18ft, 115hp rig would most likely be a value segment buyer and, even though year of manufacture does matter, it matters less in a boat versus a car and, especially if there has been no material change in the boat/motor one year to the next. If youve not already, Id take a look at it to assess its features versus your rig. That way, you are at least informed of the potential competition and can speak to it if the need arises. Best of luck.