I think it would be nice, but I'm not sure if it will be a big deal with merchants and all. I just thought I would suggest it.
First off (if this ever were considered), this would only work for items that can be purchased in NPC shops, so this would automatically exclude eggs and such, but would ALSO exclude items from the black market. All the items could sell for about half of whatever they can be pruchased for from the npc.
I just thought I'd suggest it, it seems pretty nifty to me, just 'cause you could still get a little bit of tu back if you accidentally bought the wrong item, or if you bought a piece of clothing and then decided you didn't like the way it looked on your human avatar.
So, support? Yae? Nay? If there's anything wrong with this idea, or if you have ideas on how to improve it, please say so :)