PC version (Xbox360 users can use the Complicator Fallout 3 Save Editor instead of console commands): there is a console command where you can give yourself caps in any amount, at any time. Simply press the ` (tilde) key. It's the key to the left of the number 1 on US keyboards. Type the following: player.additem 000000F (number of caps you want) and then hit enter. Hit the tilde key again and you will see an update showing that the number of caps you requested have been added to your inventory. For example, if I hit the tilde key and typed: player.additem 000000F 3500 - I would be adding 3500 caps to my inventory.
All versions: simply go to a store and look for two of the same item that are either both yours, both belonging to the store, or one in your inventory and one in the shop. If it is the item being in your inventory, sell it. Buy(back) the cheapest of the two items, it should've glitched and have given it back to you in full CND. If not, sell back then buyback. Repeat untill money count on shop is greyed out, then buy the supplies you need then, if the vendor still has money, repeat. (note: if the item is broken it will cost nothing to "buy back")
All versions: At Fort Independence you can sell power armor and laser/plasma weapons to the Outcast for ammo/stimpacks. The sold items will be at full repair in the inventory of the buyer and can be stolen back. This way you can sell a 5% lasergun and pickpocket it back at 100% while at the same time get your stimpack + access to Fort Independence reward at 50 point of sold items.