The rules clearly say that each trader gets cards of an adventure deck number equal to or 1 less than the number of the completed scenario. The cards you trade in must have an adventure deck number no less than 1 less than the scenario's adventure deck number. It doesn't explicitly say the completed adventure deck number. You can easily assume it is since you are told after paying the cost to take one of the trader's displayed cards (regardless of its deck number).
The confusing part is in the example in the box above this paragraph. It reads in part, "If the characters want a card with adventure deck number 3, they'll need to trade in cards with adventure deck number 2 or higher; if they want a card with an adventure deck number 2, they'll need to trade in cards with adventure deck number 1 or higher."
The rules don't say anything about the cards you are trading in caring about the deck number of the card you're trading to get. Which is the adventure deck number that matters, the one of the scenario you just completed (which is implied), or the card on offer you want to trade for (which is what the example suggests)?
