Something to address this would be nice

It's beyond frustrating selling something that's valuable for a meager amount, which often leads me to hoarding junk that I'll most likely never get around to using. To be fair, I generally hoard weapons that I like anyway - just in case I come across a champ that I feel like hiring and giving it to

I haven't had any problem with item selling price, my tax is nearly always at zero (unless I got prison/onyx thrones - rarely I'll go to low) and my economy still roll along just fine. It's really just a matter of perspective. What if it was the outrageous buying price that is what is ripping you off?

Yeah they had a problem with the economic model being out of control.  That seems to be fixed now with 10% selling and other fixes.  I'm ok with it.  I do like the idea of special merchants or special shops or special quests that grant you special merchants or special shops or maybe increased selling can be a Civ tech or a faction trait or hero trait.  Lots of possibilities here.

Me too.

Maybe some type of barter system, where you can trade "stuff" you have found with a certain value for other "stuff" from the store with the same value (or for a 10% markup or something like that)?

E.g. can "trade" 3 wolf pelts worth 10 each and 1 of your extra daggers worth 100 for 1 axe valued at 130...

This would allow you to get rid of your superflous items (multiple leather helmets, etc) and the random silken webs, wolf pelts, et. al. without havnig to sell them for 1 gildar; would allow you to get cooler stuff and/or things you are missing without breaking the bank; but would keep you from selling all the stuff you collect for gildar to fund your army wages, which was what broke the economic model before the huge markdown to selling prices was done.  Could still buy/sell things for Gildar under current system, but would have option of "trading" goods with the store for a less than 90% markup without having any Gildar change hands...