D&D Geekitude: Magic Item Pricing

I'm not happy with the rules for buying and selling magic items in 4th edition D&D, and no one else in my 4e group is, either. So... you can only sell items for 1/5th their default cost? But if you want to buy them from someone else, you have to pay full price? At first glance, this seems to be simple prejudice against PCs (who must have some "player aura" that NPCs can detect). I think I understand the game-balance reasoning behind it -- players would have a huge advantage if they could exchange the items they find in dungeons for the items they want of the same level -- but I'd prefer a convincing game-world explanation.

Here are a few justifications I can think of:

1. The stuff that PCs find, in dungeons, is old and beat up. If you assume that magic items suffer wear-and-tear, then they should sell for less used than new. Granted, there aren't any durability rules in D&D, nor should there be; but one could argue that they aren't necessary, because PCs upgrade equipment all the time. NPCs generally do not; they want a magic sword that will last for decades, not weeks; and one that's been smacked into a hundred stone-golems will be worth a lot less to them.
   
2. PCs generally acquire items illegally, through grave-robbery or ... well, just robbery. This stuff can't be sold in the open; it has to be sold to a fence, who certainly won't pay market price for them.
   
3. The market for magic items is very, very small. There aren't many people in the world with 325,000 gold pieces; even fewer want to blow it all on Boots of Balance. If PCs go through all the trouble to make an item, that's probably because it's really useful to them. Few others would value it so highly.
   
4. When player characters spend money to make an item, the money isn't just for materials. It's also for renting a forge (or a workbench), hiring skilled craftsmen, and (perhaps) learning a special ritual to make it.

DMs should decide which of these factors are responsible for the price differentials. If they don't apply, then players should be able to buy the equipment for less, or sell it for more. For example, if a PC makes a magic item, and then immediately sells it, it should sell for more than 1/5th the official purchase price, because the first factor isn't an issue. If they acquire an item lawfully, condition 2 won't apply, and the sale price should be higher. If factor 4 makes sense in your world, then PCs should be able to make duplicate items for less than the full sale price, and so on.