Find a respawning weapons case. For example, in New Haven, there are five weapon crates that respawn every time you load the game. Three are on the roofs above the truck stop, west staircase over the bar, and above the west gate; jump from canopy to reach it. The repaired claptrap opens another crate behind some steel plates, and there is another weapon crate north east behind a junk hill under a large canvas canopy. Note: There are a few bandits and aliens that spawn here. Collect the contents of the crates, quit the game, resume and collect them again. Sell the guns to make money. Repeat as desired.
During the mission "Looking for answers" and "Reactivate the Echo comm. System", there are five weapon caches toget, and the majority of enemies drop items. It is possible to get over $300,000 in items reach time you run through this mission. Search for the chests, but when you get to the final room before you transition to the next area, exit game and reload. This is a very easy way to complete the "How much for the planet" challenge.
During the first scene while getting the tour with the scrap robot, follow him until you reach the small campsite with the car up on the lift next to the burning tires. Collect all the guns and money along the way, then kill the enemies. Once everything is dead, pause game play and choose "Exit". This will save your character's level and items, then resume the game at the very start. Repeat this process as many times as desired.