Most people cant discern what is original, especially the bast majority of the BaT audience. If thats important to you need a pre purchase inspection. Im confident thats not happening. People are relying on yahoo comments and overlooking some very big items in some cases, like rebodies and bad tags
Hard to know how many cars are being inspected in person, but I'm guessing it's not many. There's no lead time before an auction goes live and hard to get something like that arranged withing a 7-14 day window. I bought a car off BaT earlier this year, thankfully it was a 2 hour drive away, otherwise, not sure if I would have bid on it. The seller told me I was the only one that came and looked at it, yet it got bid up to about 1.5-2x more than it should have.
PPI seems to be much more of a thing on newer cars, like hi-performance German cars and such. Guess it's easier since there are usually at least a few reputable shops in most areas.