Some items became un-vendorable after transferring from shard to shard, which is a known issue. I am unaware of why recently acquired items would have this issue other than that.
Mesanna has fixed items in the past. She can tell if its legit or not. I would recommend keeping the non-vendorable items you feel are legit easily accessible, and keep an eye our for Mesanna. When you see her, ask if she can check the authenticity.