By cheating: Use the console. Only available in the PC version. Without cheating: Always pick up weightless, valuable items, such as pre-war money, pencils, ammunition, and chems. When you reach the limits of your encumberance, repair heavy items, such as weapons and armor, in preference to dropping them. This will lighten your load, while maximizing the value. When selling items, make sure to boost your charisma and barter beforehand. Certain NPCs will give you discounts as a reward for doing quests. Find these NPCs and trade exclusively with them. Also, you can make lots of money by taking Lawbringer and/or Contract Killer. Finally, some quest NPCs will pay handsomely for specific items, such as scrap metal, though the karma rewards are usually more valuable than the meager sum of caps offered.