1. During diologue with any NPC choose trade option
2. Select to buy all the stuff NPC has
3. Close barter window with cross button without unsellecting stuff
4. Kill that NPC
5. Loot all stuff NPC had for trade
????
PROFIT

Please, fix

Edit: once you selected any weapon or armour piece for trade from NPC and canceled barter without buying, item becomes fixed and doesn't grow in stats