Me too, when buying Caravan cards off this guy, the game crashes. Nice to be a beta tester for Bethesda ain't it!
I can't find the thread documenting the fix right now, but this is what you do if you don't want to download a mod that automatically adds the cards to your deck after you purchase them.
1) drop -all- loose cards in your character inventory into a non-owned or player-owned container.
2) pick up cards, they should then be automatically added to your deck.
3) should be able to trade with merchant without CTD (unless you are doing a large transaction with many items, which is the bug I am now encountering).