I tested a method to not use up found in raid items with a mechanism backpack and the matches for painkillers trade with theRapist and did it enough times to be confident in constructing my scav case without using up my found in raid roler which I'm saving for the collector quest.
DISCLOSURE: I am not responsible for your found in raid items being used in a trade BUT I am 95% confident this method will allow you to selectively use items for construction materials.
What I did:
– Keep a few lines of empty stash space at the top of the stash
– Put items I DID NOT want to go towards the trade/hideout construction in the first few slots of my stash at the top from left to right (so top left of stash)
– Put the mechanism backpack (this likely works with other containers but I have not done more tests than this one) lower in the stash than the first empty lines which are now not completely empty because of the items I put in the top
– Put the items I wanted to trade in the mechanism backpack
– Go through with the trade/construction
Hope this helps until we can choose which specific items go towards trades & hideout contruction 🙂