I would not trust the claim that it was being sold by a "collector and enthusiast" unless most of his sales are toy trains, with items of the same vintage as yours being sold in similar condition.
How much is the price discounted? "Brand new in box and never tested" is a claim made to collectors who do not intend to operate the locomotive. The typical operator would not mind purchasing a new item that had been test run, and would probably prefer it. After all a number of "mint in box" locomotives turn out to be nonfunctional. Plus I am assuming that this is now out or warranty. You could be buying a gem. You could also be buying a shelf queen. For enough of a discount I might be willing to take a chance.
Assuming the loco is priced somewhere near full price, the first bit of information needed is what type of person you are Ponz. Are you the type of person who is going to be beating himself up six months later because he didn't take a chance? If so and you can afford to lose the money, buy it.
If OTOH you are the type of person who will be kicking himself if you open the box up and it is DOA, there might be another option IF nobody else is bidding on it. If that is the case contact the seller and tell him you are interested in the loco if it is in good condition and runs. Offer to buy the loco if he will allow you to return it at your cost if it does not meet those conditions. You might have to wait until the auction closed before he would agree, in case some fellow collector might want to bid on it. That would be a reasonable condition IMO.
