If you go to a shop and the keeper has 2 of one item (e.g. Minigun-worth 100caps and a Minigun-worth 15caps) buy the cheaper item and accept the transaction. Sell the very same item back and accept the transaction. Repeat this about 3-4 times and you will have a Fully Repaired version of the item you've been buying! Either keep your new item or sell it for a nice profit. Also, if you have an item and the shop keeper has another, at a different price, sell yours and repeat the same steps from before. I have 30 fully repaired Miniguns in my Megaton House, along with 8 Fat Men (Fully Repaired) and a nice set of other weapons. Enjoy!
P.S. Any questions, just post a message on this page, Thanks!