This is kind of difficult.
I'm not a big fan of handing my items in, even if there's a gold reward. For one, if you get gold equal to the market's price when you sell an item (IE - ~1/4 of 'retail') it's not worth it at all. Also, by handing the items in, it means that I'm unable to hand in sets I've completed that contain enchanted items, because the NPC will either take the enchanted item (noooooooo!), or WON'T take the enchanted item and I'm stuck with them until I can find a replacement.
The latter is not a huge hairy deal, but the former is kind of frustrating, especially without weighting sets. Yes, you get points and a badge, but with the gold payout so trifling it makes sense NOT to collect a large set like Zelda, since you could spend that effort collecting doom guns 8 times and end up 'ahead'.
I see that allowing users to keep an item could encourage things like tandem collections, selling whole sets, other game-breaking things like that.