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.

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84 300SD 274K
38K miles on flatplate heat exchanger and various diesel/veg blends. prior to that 4K miles on unheated veggie blends with kero and DinoD.