If the auction company garantees in writing that the bike runs you have recourse if after purchase this proves untrue. Are both fobs available with the bike? Can the bike be licensed where your Dad is at with a salvage title? Worst case he could part it out and probably double his money towards a different bike.
Having no sight glass is the equivalent to having a hole on the side of the engine. You can pour oil into the engine and start it and it will run. But it's only going to run as long as the oil stays in the engine. You still have an unknown as to what would happen two miles down the road.The OP did not say the glass was gone, only possibly. I would assume that it's existence would be confirmed before purchased.
Jeremy, did your dad buy it? If he can wait about another month and a half(after my trip ), I'll prolly have a good deal on my 08, but it will have about 48-50k miles on it then.