Cash, Eridium and DLC currencies
Farm Tinder Snowflake for Loot Train gear and cash
Repeat the Mercenary Day boss and turn unwanted Loot Train gear into cash at nearby vending machines.
Frost Bottom in the How Marcus Saved Mercenary Day Headhunter pack, reached from Marcus’s Mercenary Shop.
Repeatable after the Headhunter mission path unlocks the encounter. Each cycle is a boss fight and Loot Train chest check; sell surplus gear for cash. It is not a guaranteed source of any particular item or currency.
Stop when you have enough cash or the train no longer offers gear worth keeping; switch to another activity if your goal is specifically Eridium.
Open a guide to find other locations and see what the item is used for.
Repeat the Mercenary Day boss and turn unwanted Loot Train gear into cash at nearby vending machines.