On the difference the case would make - it depends:

If the guitar is absolutely pristine a collector might be interested but would want the case so it was all 'complete'. If the guitar has been played a bit and is more a vintage 'working' guitar then it'd sell for less but the case would be less important (in my mind anyway).

I guess knock a couple of hundred dollars off for the case. as that's about what you'd pay to replace it. I'm not a vintage guitar buff, but that's my opinion as a gear hound... :D