Rather than restrictions on trading, what about turning this into a possible service/business opportunity?
Imagine a Deckbox service that essentially acts as an escrow service.
Possible Process
Traders A and B have to submit pictures of their cards for them to both approve of the conditions. (This eliminates Deckbox having to verify conditions based on a subjective scale)
Traders send cards to Deckbox escrow address
Deckbox representative films: the opening of each envelope and each card (used as evidence that same cards were shipped as in pictures and protection from claims that Deckbox switched cards out), and the placing of cards into new envelopes (to protect against claims that Deckbox sent different cards). Alternately, something can be written into the service's terms of agreement contract that indemnifies Deckbox of any such claims
Upon verification, Deckbox sends the cards to the new owners (likely with tracking, again for liability reasons)
Thoughts on Cost
The service would cost money to cover expenses, such as shipping fees and labor costs for having someone handle the trade. I imagine labor would be around 0.2 hours (opening, verifying against trade lists, possibly filming everything, creating new shipping labels, then putting in mailbox for shipment). Assuming minimum wage may go up to $15 in the future, after employment taxes you're looking at about $16.50/hour. At 0.2 hours of labor, that's about $3.30 in labor, maybe about $2.50 for USPS first-class or $5.50 small flat-rate box (for larger quantity trades).
Labor could be split by both parties, and shipping would depend on what someone is sending. Per trader, Deckbox's expenses would amount to about $3.65 (half of labor, $1.65, plus $2.50 first-class) for smaller shipments, and $6.65 for larger ones. Add in a 25% profit margin, and you get approximate service costs of $4.56 and $$8.31. At just 10% (I don't know Deckbox's overhead costs, so I don't know what would be feasible) the pricing would be more like $4.01 and $7.31.
Worth it or Not?
This wouldn't be economical for low-value trades, but for high-worth trades, some users may find this useful. Ultimately, the cost might not be all that much higher for some traders. If any of you are like me, I decide whether to add tracking based on my level of trust in the other trader (definitely not so the other user can track where the shipment is, I couldn't care less about that). Mail gets lost on rare occasion, but I suspect the more common culprit are traders claiming not to receive cards. Deckbox would be a trusted service, so I wouldn't feel the need to spend money on tracking when sending to Deckbox. I save money on my shipping costs, and spend a little more for the service. So, for trades above a certain monetary threshold (for me it would be around $50), the cost would be minimally higher in return for peace of mind.
Those are my initial thoughts on how such an escrow service could work (I'm sure you all can think of many other iterations). The inspiration was thinking about what the problem is we're trying to fix. People tend to get scammed on higher-value trades. Lower value scams certainly aren't pleasant, but the sky won't fall if you lose $20 in cards (let alone $5-$10). This would avoid restricting traders (an issue brought up by some), create a solution for those who are interested in a trade but normally wouldn't do it for fear of getting scammed, and would add an additional revenue source for Deckbox, paid for by those interested in the service. There may even be opportunity for including some version of this service for premium accounts, but that would have to be figured out by Deckbox's accounting team to see what would actually be worth it (perhaps X escrow trades per year for free).
Anyway, just a thought and opening this up for discussion.