Just to throw my hat in the ring, Go to the black market in Sanctuary (its the purple door) and trade in eredium for upgrades. I suggest you play in CO-OP. You get bigger rewards. So instead of finding 1 or 2 bars of eredium at a time, you'll find 5 to 7. Which makes buying more items easier and faster, especially when the third, fourth and fifth items go for around 8 bars each.