I made a recent enquiry with Watchfinder about a potential part exchange. The watch I proposed to part-ex was bought pre-owned. Watchfinder have replied "At this time we will not be looking to make an offer on this particular watch without the original proof of purchase." I understand there may have been some suspect transactions in the past, but this could be problematic for anyone wishing to trade with them if they don't have a paper-trail back to the original supplying retailer/AD. I enquired about different watches about a year ago and didn't encounter this, so it might be useful knowledge.
Ant