It's important to remember that the species of wood used for the back and sides is a small piece of the puzzle. Quite hard to say how walnut sounds, but can certainly sound just as good as other typical woods like IRW if used by a skilled luthier. My question is are you able to try the guitar or have you played any guitars by this particular builder? If you can try in person, then I'd definitely just forget about what woods it is made from and assess the guitar based on sound/playability.
In terms of resale value, because walnut isn't as popular, you might lose some potential buyers... but if it is made by a well known luthier then I wouldn't worry too much. Depends how much you are buying the guitar for and how much you're willing to resell it for.
"Success grants its rewards to a few, but is the dream of the multitudes.
Excellence is available to all, but is accepted only by a few." - Christopher Parkening