IMHO selling items back would be a very useful feature – even if the refund is just 50~70% of the original cost.
This would make players less hesitant when buying expensive gear to try out, and less frustrating when they find out it doesn’t suit them.
Also, it would be a big plus regarding new end-game gear releases. I’ve spent over 4k gems to buy the sword with the highest DPS in the game currently. When a more powerful sword is released costing, say, 6k, then I’d have to start saving all the way up to 6k gems again. With a 50% refund feature, I’d have to stock up less than 4k gems and sell my current sword to buy the fictional 6k sword.
IMHO, this makes releasing end-game gear seem less like a money drain, making the upgrade process more straightforward to the player and resulting in a better user experience. Of course, this does not apply to end-game gear only, but to the entire gradual progression of the user as well.