Used SVTs in working condition sell for around 1000$ at GC, and that is the asking price on my local Craig's List.
The fact that the output circuitry is working on this amp leads me to think that a repair might be simple, and not too expensive. It might not even need any parts at all, or maybe a low cost preamp tube.
I am not knowledgeable about year and model variations to know if some are more valuable than others. I could do any repairs myself, so I would probably pay that price for it if I really wanted that particular model.
If you buy any older tube amp you are taking a chance that it will need some work sooner or later. If you will gig with a used SVT, you should probably have it gone over before depending on it anyway, so even a working 1000$ amp, (unless it has very low hours, and miles) might need some service to ensure it is gig-worthy.
I think the seller has already discounted the amp somewhat to reflect the less than 100% functionality. I like that the seller disclosed the problem in the ad. It might not hurt to make a slightly lower offer based on the known problem. What about the overall cosmetic condition? I would go look at it with 700$ in one pocket, and two fiftys in another.