I'd love for there to be a way to sell items for default prices at the State Store.

Something small like a quarter of their buy price, so there's still an incentive to trade to other users over selling to the NPC.
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I feel wasteful just destroying items when I run out of space, but feel trading post to be slow and unreliable as the site's still new and prices aren't stable.

And since there's no way to sell back as my previous suggestion(unless I'm missing it?), maybe just have a place to donate/dump items. Or be an exchanger (ie: you dumped one item, so you're allowed to pick one different item some one has left), or it could be a random item of the same rarity or lower.

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Please do tell me if I missed the Sell Back option somewhere, as all I'm seeing is the ability to Buy from the NPC store.

I don't believe there is a sell back button, so I greatly support this. 

I was just wishing I could sell back my stuff so I could keep getting more important items, instead I ended up destroying things. 

Definitely support. Just had to destroy something myself to make space.