My opinion is that the rule about selling magic items for half value is kind of dumb in most campaigns. Magic items don't really wear out, so the whole "previously-owned" discount doesn't seem like it should ever apply.
The only real exception being campaigns where the gameworld has magic item stores, where magic items are so commonplace and ubiquitous that merchants can actually make a living by keeping racks of +1 swords and the like around waiting for someone to drop in and buy one at market value. In that kind of world, I can totally believe that an independent adventurer could readily find someone to buy an item, but would rarely get more than half-value for it (since the merchant's going to have to mark it up for resale). It'd be like a used CD store or something: bring in an item, and we'll give you this amount in gold or this slightly higher amount in trade credit.
But then, I've
never played in a gameworld that had magic item stores, so I'm left wondering, who exactly is it that keeps getting these items at half-price? Why is it that the PCs never stumble into a guy who's just dying to unload an item at half of its actual value? How is it that some settings can say "magic items are rare and extremely valuable, so people tend to hang on to them like grim death rather than sell them" and then turn around and say "but if you ever DO try to sell one, no one will ever pay you more than half of what it's worth"?
So, my suggestion is that you abandon the half-price rule, unless your gameworld has a well-developed Used Magic Shoppe retail network in place. If you don't want the PCs to sell an item, don't let them find a buyer. If you don't care if they sell it, let them unload it at the market price (I mean, isn't that what a
market price is for?

) or maybe a bit less if you're feeling catty. Let crafters sell their products for full market value or higher. Let NPCs try to commission items from PC crafters, to balance out the PCs who are trying to commission items from NPC crafters.
Or if you want to spend a lot of game time worrying about this, you can even go in the direction of establishing dice rolls for each step of the sales process...Gather Information to find potential buyers, opposed Profession: Merchant or Diplomacy or Bluff checks with Sense Motive as you haggle over the price, followed by some kind of adjustment from a base figure (80% of market value, give or take 30%?) to determine how much it sold for. And then be fair, and apply that same process in reverse when a character wants to buy something, so that the bargains of a lifetime aren't exclusively restricted to the NPCs.
(Personally, I wouldn't go that route, because...uh...it's incredibly boring for anyone not directly involved in the sale, and not exactly thrilling for the characters who ARE directly involved. But your mileage might vary. Perhaps haggling and shopping and the like are a veritable ocean of gaming fun for your group.)
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i can't imagine how it could be, but hey, people are weird
ryan