I've always enjoyed consumables in other mediums. A scroll for example is nothing more than a spell in written form, waiting to be activated whereupon it disappears from the page. It requires its user to have deciphered it and have the knowledge to complete its activation, either by having knowledge or spellcraft or the ability to use magic devices.
So a scroll could easily changed to a painting, cuneiform tablets, old computer punch tickets, etc. It might be a song, etched on a disc, that must be played and harmonized with. Ok, maybe that last one is off, but hopefully you take my meaning.
An even easier one is potions. You drink a spell. Why not eat, or smell, or inject? There are lots of interesting mediums to deliver a consumable.
Also, you're dealing with a trading post right? Trade! Folks might not need gold coins out on the frontier; instead they might need a band of mites and their giant spider removed for that masterwork shield, or the feathers of a griffin for the scrolls you're interested in. If the trading post really is on the verge of a wilderness, check out the rules on Monster Trophies for some inspiration on what might have possible value.
For your players with encyclopedic knowledge, challenge them. Drop some magic items that you only give the description of. Personally I like rods for this. A rod can look like a simple Billy club, a king's scepter, a club or a mace; there's a few different things that a rod might be. What it ends up DOING might only be hinted at by its appearance.
As others have said, make unique items and NPCs. Take a new spell, or one that's kind of niche/obscure, and make an interesting new Wondrous Item out of it. For example there's a spell no arcane caster in my games would ever take, called Miserable Pity. Enemies that fail their save refuse to attack someone under the spell unless the recipient themselves makes an attack.
What about a locket or a robe or something that can be activated a couple times a day to give the user 3 rounds of the spell? The item can look perpetually beat up or worn, but no matter what you do it never gets any cleaner. This might be an interesting, niche item to drop at the trading post on the eve of a mission where the PCs need to infiltrate an enemy lair with guile instead of force.
As for NPCs, try to drop a little backstory on WHY someone is trading versus just exchanging money for magic items. If a traveler has a magic sword and wants to trade it for the cleric's armor and some divine spellcasting services, maybe he's a fallen paladin looking to redeem himself; perhaps he's a paranoid former rogue who is looking to ratchet up his security; maybe he's even not a mortal at all but a divine outsider who will use the armor, enhanced by a collection of random, divine spells, to construct a golem of unbelievable power!
Then again... your players might not want any of this.
If you have players who've memorized every item in the books, maybe their fun in the game comes from the mechanics; from knowing everything and how it all works. It might be that, if you have a vegepygmy shaman 1 offering preserved mushroom caps that must be squeezed while the "reader" of the "scroll" taps an accompanying rhythm to complete the spell... your players will just roll their eyes and say blandly "so, it's a scroll of Bless? Great; I pay him 25 GP" and call it a day.
Talk to your players, see what they're actually interested in. No need creating an interesting trading post your players are going to use as a rest stop between monster fights.
