It strikes me when reading through old Dungeons & Dragons manuals that back in 2nd edition AD&D, there was practically no such thing as buying and selling magic items. By contrast, in the current edition of the game you can travel to any city and expect to freely buy and sell any magic item save for the few most expensive things in the game. (Drop a comment even if you don’t read the entire entry, I’m interested in seeing what people think about this.)
The difference really changes how treasure works in the game. AD&D works like Diablo II, where huge amounts of gold are largely ineffectual for buying magic items because the kind of people who value magic items tend not to accept established amounts of currency for them. Magic items are so rare and difficult to create as to have no established price.
Come third edition, and any magic-using NPC or player character with the appropriate training can create magic items from scratch; similarly, by the rules at least, any magic item worth 100,000 or more can be commissioned or purchased reliably for gold, while players will find that they can consistently sell any such item for half market price.
The big difference is that unwanted or unnecessary magic items now suddenly gain special value, because you can reliably trade in stuff you don’t want for stuff you do. A spare +1 sword is no longer just a spare sword; it’s a convenient portable 1,000 gold piece ingot redeemable at any nearby town. Because they’re now tradeable for currency, anything that’s worthless to you in practice is still valuable, and anything you desire is attainable just by hoarding enough worthless junk and dropping it off at the city magic shop between adventures.
This can be a good thing, or a bad thing. Yes, it opens up new options for players, but I can’t help but feel that the game loses something by making magical equipment a commodity. It’s less heroic; I don’t remember the part where Frodo flicked through a magic item catalogue and got a trade-in on a bunch of enchanted amulets. It makes more work for the players; someone has to keep a list of the items nobody wanted, then add up the prices whenever they’re to be sold. It gives players a reason to look through the magic item list like a mail order catalogue, which makes items less mysterious. It makes magic feel more common, such that you can reliably stock up on consumables, base character builds around items, and remove the reliance on party spellcasters for buffs and cures. Finally, it removes the interesting hodgepodge element to the game, where an item is an item and you can take it for what it’s worth or leave it.
tl;dr: Do you think it’s better when you can’t buy and sell magic items? Discuss!