A lot of dealers say they won't verify internals, because they are afraid of taking the gun apart, as they don't know what to expect.
It is amazing that he's walking away from a sale with you by being this way tho? You could copy and then print off the info that says what is required for it to be an all matching luger. If he still says he won't let you look at it, you could say, okay then, is there a 100% moneyback guarantee that its all matching; to include these parts?
Another friend says that he tells the owner he knows how to take them apart and put them back together and that usually does the trick
Ed