You Run Barter Town 15G
Sell 10,000 caps worth of goods.

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While this achievement can be acquired over time, it is obviously easier with a high barter level. If you want to power through this achievement, you can use the following method:

Find a merchant.
Sell the maximum amount of goods to a merchant (equal to his / her caps on hand).
Complete the transaction.
Buy back your own goods.
Repeat the last three bullets until you are out of caps / get the achievement.

If you are having trouble finding high value goods, most unique weapons sell for a lot of caps. You can also sell Power Armor for a hefty profit (kill NCR Heavies or BoS members for it). Finding all the snow globes, and selling them to Janet in the Lucky 38, will net you 14k caps which would make it very easy to use the above method. Alternatively, you can simply wait until after "The Courier Who Broke the Bank" and get the achievement quite quickly using the above method.

If you want to attain this achievement in a gradual fashion, then you can get this by simply looting everything you kill, combining the items by using your repair skill, and then selling all the goods when you get back to town. Remember that your influence with a faction determines the buy and sell prices of its merchants, so higher favor = less time and money!