Don't leave cards or decks unattended. "Just for a moment" is the catchphrase of the forlorn gamer. You wouldn't leave a stack of cash alone with hundreds of strangers for just a moment. Don't do it with cards. #MTG
Don't leave cards in your car. In the trunk/boot is better than the cab, but it's still not good. Parking in a lit area under a camera is better, but still not good. Yes, it's obnoxious to carry cards everywhere. Less obnoxious than replacing them. #MTG
Do not rely upon other attendees for your security. They may be strangers. They may be friends. Even if they don't intend malice, they won't be as diligent as you are with your cards. Would you leave a pile of cash, even if that person was watching? #MTG
Do not check bags with your expensive cards; carry them on. Stuff goes missing at airports; sometimes whole bags, and sometimes just contents. And some crooks make a point of lifting off baggage claim before you arrive. #MTG
Do stay stay at shoddy motels. It's very very easy to break into a hotel room, either physically or via social engineering. It's easier at shoddy motels where employees often don't care. (Plus, your back will thank you for the comfortable beds.) #MTG
If your room has an inroom safe, use it. Do not use 1234 for passcode. If it does not, store cards that you brought somewhere folks are unlikely to look. If your value is high enough, ask the front desk for maid service only while you're present. #MTG
Do not chat or brag about the value of the cards you brought, in the venue or at local food places. Do not flash cards. (You wouldn't flash stacks of cash around, right?) Be aware of surrounding and potential followers when heading back to your room. #MTG
Lack of coverage for Fire/Flood/Theft/Earthquake
Lack of coverage if cards are taken out of the house
Lack of coverage without individual pictures
Reimbursing "fair value" (if cards increase in price, or if cards are unique some way). #MTG
Lock. Your. Doors. #MTG
Choose a public place. Choose a place that is in a population center (no back roads). Choose a place with security and cameras. Police station parking lots are a great choice. Do not meet in the middle of the night. #MTG
Consider ways of securing your value before meeting. Wait to withdraw cash from an ATM until after meeting and seeing cards. Consider digital transfer options. If the other person protests your safety, LEAVE. #MTG
If one person unexpectedly turns into multiple at your meet, leave.
If they try to get you to follow them, leave.
If anything makes you uncomfortable, follow your gut instinct and LEAVE. #MTG
After you leave, especially if you left because of feeling uncomfortable, do not go directly home. Stay in well lit populated areas. If you have cash, go to a bank and deposit it. If you believe they are following you, call 911. #MTG
Consider setting up a dead man's switch. Let a friend or roommate know WHERE you're meeting and WHEN you'll be finished. Tell them that if you don't check in by that time, to call 911 and provide that information. #MTG
If they pull a knife or gun, give them any items they want. Your life is worth more than cards. Follow directions unless those would incapacitate you (eg cuffs, get into a car), in which case run towards security while yelling. #MTG