By using this tutorial you should get free/very cheap repairs and a few extra caps. Just follow these steps:

1. Go to a shop keeper and trade with them (make sure they sell weapons)
2. Make sure the shopkeeper has at least one of the item you want repaired and you have at least one of the same item.
3. Sell your item to the shopkeeper
4. Buy the item back (nothing will happen this time)
5. Sell the same item back and then buy it back again
6. When you buy the item back you will notice that for you, it is fully repaired. (If it is only partially repaired, repeat until it is fully repaired)

You can now sell the fully repaired item back for a price that is higher than the price you bought it for. It will remain the same condition and price as when you first sold it in the shop keeper's inventory, but will be fully repaired and worth more when you own it.