I would think that for the novelty factor alone that a 3.5″ rim would certainly attract a premium above that of a 3.00″ rim. What that premium is though is almost impossible to say. A seller who knows the rarity of these wheels is certainly not going to give them away in this day and age I wouldn’t think, and like most rare things the selling price is what someone is prepared to pay. If you don’t buy then I am sure some other equally keen R afficianado will eventually.
Would there be any advantage using one on the road? Probably not I would say, apart from a great talking point at meet ups.
The pictures show however that the seller is not totally correct with his sales patter re: the silver rim. These are from the wheel that Ralf Buhl got hold of a few years back.
Good luck with your negotiations. I look forward to hearing how the deal goes.