Sell 50 guns to a shop

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This should also be achieved before the end of the first playthrough. Simply collect loot dropped from enemies, found in chests, and rewarded in quests and then sell it to a vendor. Selling med-kits and other loot will contribute to this achievement. Progress can be tracked in the challenges menu.

A way to get this very fast would be to keep buying and selling everything from the same vending machine.