Descent 2nd Edition Community Rules Reference Guide
Yes, it's in the Community Rules Reference Guide under 'Trading'
Yes, I put in in there
Here is the official FFG answer from Kara:
Q: Can heroes trade used search cards that lay facedown? I didn't find any contradictory information, but still I have doubts about it (it may be important for Treasure Hunter abilities).
A: Yes, heroes may trade facedown Search cards.