Just like Fallout 3, there is a trading exploit to earn basicallyinfinite amounts of caps, items, repairs and ammunition. Just do the following steps:
Step One Find any shop-based trader and sell him anything in your inventory that you don't need and complete the transaction. This will increase the amount of money he has.
Step Two Find something in your inventory that you have duplicates of or something that you and the shop keeper both have. Sell an item or two to the trader so he has two of them.
Step Three Make sure that the two identical items are of different values once they are in the trader's inventory. The bigger the difference in value, the better.
Step Four Now purchase the cheaper of the two identical items and complete the transaction, but do not leave the trade session. Look for the item in your inventory. The item should have gone up in value dramatically and should be 100% repaired, even though when you sold the item initially it wasn't! If it isn't 100% repaired for whatever reason, sell the item back to him anyway and repeat steps 3-4, trying a different item if necessary.
Step Five Now you can sell the same item for a high price and buy it back at a low price until you have drained his money completely. If you want, you can hold onto the item as it will be fully repaired -- simply keep it on the final round which will leave him with a small amount of money. Either sell the item back to him again once he is out of money, or if you are keeping it, sell him something else of high value and he will owe you that amount. The transaction price will go dim and remain at whatever amount of money he has left.
Step Six Search through the trader's inventory and find stuff you need. Stimpaks, ammo, miscellaneous item, weapons and armor are all good. Eventually the transaction price will light up again and you may owe him a few caps depending on how much of his stuff you added to the deal. Complete the transaction.
Step Seven Wait three days and he will have new stock and around 650 caps. The longer you wait, the more robust his inventory will become and the more caps he'll have for you to take when you're ready to do this all over again.
Note: Be sure to save while using this method since the game is prone to crashing during the transaction.