How much damage could have been done...
 

by someone running a 300SD on unheated WVO and WVO/diesel mix for a summer?

I am looking at one that looks good otherwise -- lower milage, and is a good price, but I am a little uncertain about this aspect.

Probably pretty minimal damage. Depends on where it is located. I'm sure that the injectors are plenty filthy, but nothing that diesel purge and a good italian tune up can't fix.

I ran my '81 300D like that for a couple months this summer and I have seen no adverse effects. It is running on pump B20 biod nowadays.

If it still starts pretty easy when cold the rings are not coked up killing off the compression. Even if a little coked they can be cleaned out I think without taking the engine apart.
The injectors may be dirty. It runs okay so the injection pump is probably good. Little if any downside I can think of at this time.

I did it for several summers with no ill effects. Many do. Knock on wood the injector pump could poop tomorrow but I wouldnt hesitate to take off cross country anytime. I don't feel any less confident in its longevity.
Ask the seller what he did to de-water the oil. Open the fuel cap and look for built up light colored goo. A few questions about how they filtered and blended will either put your mind at ease that they are well read or raise alarm bells if they dont know anything about it.
My thoughts are if the seller is experimenting with fuel blends then the car is probably something of a hobby for him and the car is potentially getting better maintance than an owner that wasn't a tinkerer.

It makes a huge difference WHERE he was doing this. A point you neglected to mention.
What was his blend ratio?

Did you take it for a test drive?

Wisconsin, but summer pretty much anywhere in the US is warm, eh?

I think he just ran it straight, at least some of the time.

I haven't driven it yet, but now I am purchasing a different one, so I'll never know.

If it was properly de-watered and filtered, and the weather was warm enough, possably less wear than on diesel.

Personally, I wouldn't touch a car that's had WVO in it unless it was extremely cheap. Why take the risk, there are plenty around that haven't been abused.