"You run Barter Town" achievement guide.

Sell 10,000 caps worth of goods.

This achievement is worth 15 G

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.