Congrats on your new aquisition,
First,Since you said it is not model marked in the crane Its probably a "Pre" Model 19 Combat Magnum not a no dash.
If you post your serial numberr we will know for sure.
Pre models were made prior to the implementation of model markings.
Each subsequent revision got the next dash number,
I am actually shocked a dealer would offer more than $100 for any Smith revolver especially one with a poor finish.
Not saying your gun isnt worth $400 just that dealers dont pay alot for used guns with finish issues.
I recenetly purchased this "Pre" 19 Combat Magnum / no box for $625,
As a refinish costs over $200 and will never be original I think yours is actually priced right at $400 .
Also, I cannot tell from the 1 grainy photo if the finish is beyond rescue but if so and it bothers you a factory refinish would be OK in my book.
Thats me,
Good luck either way and thanks for sharing.