I think I've got this one figured out. It seems to only occur if you try to purchase a playing card that you already have in your inventory. I have not seen this issue anywhere else, it only occurs with the merchant at the outpost. I can purchase everything else from the merchant, including cards that i don't already own, with no problems. (I'm running Win 7 64bit).
Thanks for confirming this guys. I suppose I should've mentioned that all the cards in the merchant's inventory caught my eye. It was also a bummer because I was buying more 20 gauge shells and some .357 rounds as well as some 5.56mm (with some nice armor piercing rounds as well as some hollow-point. Yay multiple ammo types! Thank you Obsidian for the classic and awesome stuff!!) and then selling any crap I didn't need. I hope this issue is resolved soon, because it really doesn't make sense that you could have duplicates of other items but heaven forbid when you get multiple cards of the same type you violently get forced to the desktop. I just went ahead and said 'screw you' to the merchant as he walked by me. I just went pass him and headed back to Primm to conduct business. If this issue gets fixed soon, I won't have to fear picking out and buying the things I want when I talk to the guy. I have loads of ammo and other junk, and somehow the same kind of card crashes my game. lol I guess I'll try only buying and selling ammo this time and maybe some water and totally leave the cards alone until the issue is fixed.
However, I never encountered this problem before in Fallout 3 or Oblivion, strange. Maybe I'm either lucky or just missed out on the fun. lol Well, I think this conclusion makes sense. Inventory bugs have been reported before, so I guess I shouldn't have been surprised.