There are several reasons for DAO revolvers.
Eliminating the SA capability forces the owner to use the gun in DA mode which is most likely how it will be used in a true crisis. This forces the owner to practice in a way that will actually help learn to use the gun the way it will be used if it is needed to extract the owner from a jam.
Eliminating the possibility of your cocking the gun in an emergency means you will never be faced with the choice between decocking a loaded revolver with a full load of adrenaline coursing through your system and putting away a loaded and cocked revolver to deal with later.
In addition, it reduces the chance that you will be accused of firing accidentally in a situation that you claim is self-defense.
As mentioned, since the gun can't be cocked, a hammer spur isn't necessary and that makes the gun more snag proof.
I'd try to talk them down a little. $429 for a used DAO GP100 with fixed sights seems a little bit expensive to me. I used to see these guns for well under $300. Prices vary from one location to another and admittedly I'm talking about prices a few years ago, but $429 still seems high.