Cheeky little trick to farm Eridium, that's quite fun to do and effective. (Saw it in one of the many BL2 videos I've been watching recently.)
Basically, you take a high level character (my highest is Level 52), and open the Golden Chest in Sanctuary with a Golden Key. Take the items from the chest and put them in Claptrap's Super Secret Stash.
Then log on with a low level character you want the Eridium on, and retrieve the items from the stash. These items will vendor for a massive amount of cash relative to your low level character's wealth. (The items my Level 52 was getting out of the chest vendor for around 40-50K, and a Level 15 will generally have 2K on them.)
Once you've vendored the items, go to the slot machines in Sanctuary with your big wedge of cash, and merrily spin away. (The cost of the slot machine spins scale with level, so at Level 15 it's about $250 to spin the machine.)
The slot machine has the following probabilities of spinning an Eridium winning combination.
SINGLE ERIDIUM BARS - 4 Eridium - 2.56%
DOUBLE ERIDIUM BARS - 8 Eridium - 0.77$
TRIPLE ERIDIUM BARS - 12 Eridium - 0.23%
Obviously with the normal bankroll of a low level character, you'd run out of cash long before you could reliably harvest a decent haul of Eridium, but with the high level character twinking down nice purples to vendor, cash is effectively unlimited.
I've done it on my two new low level characters, and I also went the extra step and traded a couple of items with Mrs Hearthly for her to vendor, here she is rinsing the machines in Sanctuary on her Level 17 character. (It costs $333 to spin the machine at Level 17.) She's hit the double bars on the right hand machine for 8 Eridium.
Obviously you're limited in what you can buy with Eridium, but the nice big backpack and plenty of extra ammo capacity of all the different weapon classes is a very pleasant quality of life improvement.
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