...even with a fake watch.
It's very common to see genuine papers and boxes for sale on various websites/auctions. Some of these sites try their best to stop sellers from offering paperwork, etc. without the watch specifically because they realize these items are being used to pass fake watches off as real and they don't want to get caught up in the middle of a scam. But they usually will only stop the sales if the seller has a history of selling empty boxes...if it's a one-time deal, they will often let the sale continue.
So, unfortunately, papers don't always offer proof of product.