Deckbox should avoid all involvement in these cases as much as possible, and treat them the same way they would treat a "cards in worse condition than specified" claim. Unwind the trade, everyone gets their cards back, move on. There is no way to prove intent, there is no way to prove authenticity (there are tons of cases of people on eBay buying real items and claiming fakes, then sending back fake items, for example), and it should be left to the individual traders. It is not the admin's job to authenticate cards, they are there to make sure trade issues are resolved and the best resolution in these cases is going to be the unwinding of the trade.
There is no situation where a trader should retain someone else's cards when a trade has been unwound. It leads to a lot of negative territory and there really is no excuse for it.