I'll start with an easy one.....Fallout 3 players will surely remember this one.

Go to any vendor and check his/her inventory. If you see that the vendor has 2 or more of the same weapon (10mm pistol, caravan shotgun etc.) then buy the cheapest one of the 2 and sell it right back. Then without exiting the screen, re-buy it and re-sell it again. Do this about 2-3 times and suddenly you'll notice the weapon is not only at 100% condition but you can also sell it at the max price. Keep buying and selling it to buy everything you need from the vendor for free!!

Alternatively, you can sell one version of the same weapon that the vendor has and the trick will still work. For some reason, say you have a 9mm smg and the vendor has 2 or more, when you'll buy one it won't show up in your inventory. You'll have to either drop the one you already have or sell it to the vendor in order to be able to sell the "glitched" version.

For some reason in NV, this trick doesn't always work with armor.

This can also be a very efficient way to "repair" your common weapons. Simply sell the vendor your damaged weapon and keep the "glitched" version.