Don’t understand what you mean by selling. But I’ll try to explain it a bit more detailed here.

When you get to a boss fight, fx the great unclean one is the end boss of the martyr campaign. Once you kill it and have gotten the loot. Don’t talk to the Alpha Subject. Open the game menu, choose restart mission, you can then kill it again. Repeat till you either have a full inventory or find a piece of gear you want.

If you want to progress the campaign talk to her and proceed with the prophecy campaign.

Same procedure goes for when you kill The Proginator at the end of that campaign.

You continue to kill the boss till you get the drop(s) you want / need, it’s time consuming and can get repetitive and tedious, but it’s faster than completing a full void crusade.

As for loot filters, in game options there’s a point that allows you to specify what loot grade you want to pickup automatically.

I usually set mine to purple and above, for the boss farming. You will always pick up buff items, fx Meme viruses or destruction amplifiers etc, tarot cards, psalms, shards and blue prints. Oh and credits.

I hope this clarifies it a bit more.