Can you trade items between characters in Frosthaven?
No. Items cannot be transferred or traded between characters at all — not at the outpost, not between scenarios.
Each item card belongs to one character. The rulebook says plainly that items cannot be transferred or traded between characters, so no passing a potion to the Drifter before a fight.
What you can do is own more items than you can carry. Everything a character owns sits in their tuck box, and you choose what to bring into each scenario up to the item limits.
The other limit that catches people: you cannot own two copies of the same item, and two characters cannot own the same physical copy.
“Items cannot be transferred or traded between characters.”
Rules change with errata and with digital patches. If your copy of the book disagrees, the book wins — the official Frosthaven FAQ is the next place to look.