Buying and selling magic items
I'm playing in a long-established 1st edition campaign (interrupted occasionally by diversions into Champions, Deadlands, and Living Greyhawk) and the DM, I think, has got it right.
Our characters rarely sell magic items. The high-level group runs across a large number of +1 weapons, armor, and other low-level magic items, but due to the sheer number of henchmen, hirelings, and followers, nearly everyone of these low level items gets doled out to one of these followers.
The lower-level group tends to run across items that the group can't use because they already have items of equivalent power. As a result, these items tend to get traded for training costs, or for magic items that the group can use.
Neither trading nor selling is guaranteed, though trading them off for training costs always works. First there has to be someone interested. Then this NPC has to have enough money to buy the item. This is usually possible only in large port towns or cities (usually national capitols). It can take a month or two, and if we're seeking to barter magic items for other magic items, it is very likely that we don't get what we want (unless it's something very common). We get a better deal when trading, but selling usually means that we're getting 1/3 to 1/2 of the "book value" at best.
So far, no PC has ever succeeded in buying a permanent magic item or a wand, rod, or staff. +1 weapons are common enough that it might be possible to buy one, but no one has ever felt the need.
Tarek