Valahad is almost level 7. At this point, his vanilla holy symbol+1 is of decreasing use for him, especially if he gets a better holy symbol as his next reward(hint hint). Why not get a restful bedroll or some alchemical item instead? Y'know something he'd actually might use.
That exemple is the one I wouldn't agree with. That's is the kind of item that should be sell at 20% of their value. It is how D&D have planned the thing.
But now, assume you have a Pacifist Healer with a Chainmail of Sacrifice, then suddenly, the rules change and his Healer's Lore class ability doesn't affect anymore his healing power and half his power just got a nerf. Now, he want a Healer's Chainmail to offset that nerf.
Or let's say you have a Cloak of Distortion +1 for your archer and suddenly, the cloak is nerf and it doesn't do the job anymore and a Cloak of Walking Wounded would now be most useful to compensate the protection you lost.
That would be two exemple I like the idea of this proposal. In both case, I think Kalidrev add-on is a good one:
The item traded must be in the same slot. And that rule would be great in a few cases:
1) A new item get out and fit so well my concept
2) I've been nerf and I want to compensate that
3) My item have been nerf and is now mostly useless to me
4) I just did an overhaul and an item is no more useful to my character.
And with
the only once per tier, I think any abuse would be prevented.