As noted, you can either believe the seller, do your own research, or develop a personal network that you can contact for advice. Many collectors look at the listings on OF, and if something jumps out as being incorrect in the listing, they will usually ask the seller to correct it. That said, there are no guarantees and each buyer needs to be responsible for their own purchases. Sold items on eBay are a good way to research market price, but you need to compare watches that are in comparable condition, which takes some experience.
I did not find the word "original" in that listing. I am not saying that any parts are not original, but the seller does not claim that to be the case. "Authentic" and "original" do not have the same meaning in the context of vintage watches. It's important to learn to review photos and listing text very carefully and pay attention to details.
