This will give you all the steps to getting unlimited coins in Fallout 3.

1. With no items with you go to a shop keeper and look at the items they are selling.

2. Look to see if they have 2 laser pistols, Combat armor, or leather armor. If so one will be cheaper than the other, so buy the cheaper one.

3. After you buy the cheaper item sell it back to the store keeper. You should receive more money than you paid for it. (keep buying and selling it back).

4. After you continually do this for a while you will have a high amount of coins without having to sell anything.