Step 1 - Find a vendor with any type of ammo.
Step 2 - Choose to purchase all of any type of ammo so it gets sent to your side of the screen during the transaction (Don't actually confirm to purchase the ammo)
Step 3 - Click on the ammo that is now on your side of the transaction screen and choose to give 1 ammo back to the vendor.
Step 4 - Repeat the above, but this time give all the rest of the ammo back. (with different ammo)
Step 5 - Send all of the original ammo back to the vendor as well.
Step 6 - If done correctly you should still have 1 of the ammo on your side of the transaction screen that you can infinitely click on to get all of the vendor's caps.
Step 7 - After selecting your single infinite ammo, you can purchase all of the additional ammo that the Barter has created from this process. Do this repeatedly to get caps infinitely faster. Note: This glitch works with ANYTHING as a substitute for the 2nd ammo, as long as you take all of the second item from the vendor.