O.K. everybody, to get seemingly infinite caps all you need to do is:

find a merchant that has two of the same weapons or armor in different condition (Flak n' Shrapnel's is the best.)

Buy the cheapest one

Sell it

Buy it again

You will now notice it is in full condition, sell it and it will be in its original condition.

Keep buying and selling the item to get full condition items for crappy condition prices and, Voila! His caps are gone and you have them all, this is great to get your caps back on a big shopping spree.

From this technique, I have over 100 mini nukes, 200000 5 mm. bullets, 800 stimpaks, and full condition weapons galore! Hope I helped!