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The You Run Barter Town achievement for Fallout: New Vegas is worth 15 G

One way to get this is to get 10,000 caps worth of stuff TOTAL in your inventory. Then go to the Gun Runners (near Freeside), and save the game. Sell EVERYTHING in your inventory to the vendor, and once you get the achievement, reload the save.

*Note* I believe that you are initially only capable of selling 8000 caps worth of stuff to the vendor, since he only has 8000 caps. So, all you have to do is buy back some of your sold items (or his items, it doesn't really matter), and sell them once again to the vendor until you see the achievement pop up