I'm curious if they have or might reach a point whereby you'd need to open up the watch and look at the movement quality?
To expand on this, serious watch collectors will keep service records from authorized dealers as proof of authenticity. For example, many models of Rolex have “service” parts that differ from the parts originally used in the model. Pricing general follows the hierarchy: original parts > oem service parts > non-oem replacement parts. The pricing gap between each of those steps can be in the tens of thousands. So if eBay authenticates a Rolex as original and you later on find out it has some non-oem replacement parts, you’ve likely lost a significant amount of money on that transaction.
It'd help us to open the watch, naturally, but we can still rely on pics. Make no mistake though — the watches can be 99.5% perfect, but (at least for the watches we're handling, which have high values) the compromise in manufacturing processes is spotted one way or another
It's worth pointing out that the discussion about replacing pieces of a fake watch with authentic pieces (making a so-called 'franken' watch, as rep watch connoisseurs call it) is a totally separate discussion