Loot picker upper guy

[Rakushun]

I just unlocked the weapon seller talking to me about how he'll sell me items I didn't pick up during missions, and it seemed to be its own tab. But after this point, I've just...never seen it. I even purposely missed chests to see if it would happen and it doesn't. What's the deal?

[qubit]

If you lose a fight and accept the loss instead of restarting or using a checkpoint, you'll keep some of the loot but some will be marked as "lost"; you can them buy that lost loot from merchants if you really want it.

Some equipment reduces the amount of lost loot if you end up losing a lot and don't want to have to buy some of the good loot you found.
Remember, abandoning a mission counts as a loss: if you want to grind a boss to try to get their drops but don't want to play the entire mission, you can rush them and just abandon the mission from the menu. Equipping these means you don't have to check to see whether you need to buy stuff back.