In my experience, the easiest +3 items to trade are steel bolts, low tier hand armor, and elite cav shield. You won't rip people off, but if you offer one for a fair trade that shit will get scooped up quick.
I change builds frequently when I retire, so I don't have a "MUST GET" loomed item. But i really should get my hands on some +3 rus cavalry boots. I use them 90% of the time...