I don't really care; what I care mostly is about NOT having a bunch of armors & junk in my inv and selling them for hundreds/thusands of golds.
I think that traders shouldn't buy all our trash commonly; is really senseful to buy all the trash any adventurer bring you to your store? They should be sellers so, if by any mean they keep buying all our trash items, they should at least, buy them at 10% of its price at most. That way, if an iron helmet's cost is 100g, they'd buy at 10g, and sell from 100 to 300g depending on our skill.
That should keep us away from walking with docens of trash items in our inventory (I've always asked myself where did my character kept those three sets of armor, hundreds of ingredients and docens of potions if I hadn't even a small backpack)
i noticed in OB that like the general merchant would buy everything but if you sold items such as swords/armor to the weaponsmith guy he'd pay more for them. if i remember correctly the npcs do keep the items you sell them. what would be neat is if you for example sold them an iron chest piece, then you sneak past him, go to his cellar where he keeps everything he owns, steal it, then sell it back to him would be AWESOME! especially if it's a super rare, very expensive item