"...I'm a player who LOVES the Organized Play use of Traders. Traders allow you to tweak your deck in ways that may not be possible in normal class deck play, and they let you tweak your deck composition. If you're playing a wizard with a weapon slot; feel free to use Sunburst Market to trade the weapon for a blessing or an attack spell. Virtually anyone is also happy trading one of their basics for a Cure spell, even if they're not a spellcaster (it's still a strictly-better Potion of Healing). If you want to switch the basics in your deck (lets say you just got an Ultimate Add-On deck, or you just changed your mind) then trading for something that will banish itself (like a spell or potion) is a nice way to 'fix' your deck after the scenario, too.
Some characters I might be more happy to play because I know I can mess with their decklists via traders. I can trade for the basic Blowgun weapon from Mummy's Mask (through Sunburst Trader) to give Rooboo a means of using her 1d10+4 Stealth skill in combat, for example. Or give Yoon another blessing.
Some OP rewards modify the cost of going to traders, and several can be used for 1-for-1 trades. Give Skizza an Alchemist's Kit via Falsin Deek (A largely unfair, but possible, combination)? Some scenarios may reward card types too; I often go to the Ghoul Market to prepare myself with more armors for Siege Deck scenarios ("Defensive Stance"), since you're punished greatly for taking any damage that isn't reduced to 0, but rapid explorations are no longer a big deal.
I'm a big fan of the OP method of Traders, and I really hope they somehow appear in all future seasons. The ability to tweak and optimise your deck just that little bit more opens up so many new strategies and fun decision making potential.
... GMs, storeowners or the like may be able to give advice or cheatsheets. The point that Traders are weaker in general if you don't know the set, whilst true, is a statement that also applies to character selection or even boon selection/deckbuilding; and I wouldn't argue it's a particularly new concept. It's just that normally in OP, characters worry about what banes they'll encounter, rather than what boons they can temporarily grab. Even basic boons can be themed for specific tasks/banes that may not be relevant in every set; just look at things like Magnifying Glass, Book of Garund, Bottle of Rum or Blessing of the Elements."