Find a respawning weapons case. For example, in New Haven, there are five weapon crates that respawn each 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). The repaired claptrap opens another crate behind some steel plates, and there is one more weapon crate north east behind a junk hill under a big canvas canopy. Note: There are a couple bandits and aliens that spawn here. Collect the contents of the crates, quit the game, resume and gather them again. Repeat this until you get your desired weapon and sell the extra guns to get as much money as desired.
The best area to get money and weapons is in the mission "Looking for answers" and "Reactivate the Echo comm. System". There are five weapon caches to be had, and the majority of enemies drop items. You can get over $300,000 in items per run through. Search for the chests, but when you get to the last room before you transition to the next area, exit game and reload. This is a simple 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.