Ridley's Cohort said:
Any heroic and Good character would be more than a little hesitant to allow an evil item to run loose in the world. The true value of an evil weapon is zero (or less) from this POV. I can understand that Greedy aligned PCs may legitimately have a different opinion.
No character that a paladin would associate with can use them, and selling them means giving more power to someone who is certainly evil.
Is it? If some evil group spends its equipment budget on EVIL! weapons of some type (maybe Unholy weapons), are they really that much better off than if they went with conventional, non-aligned abilities? Are crazed cultists with Unholy daggers that much more dangerous than ones with Flaming, Shock daggers or Wounding knives? Any other surplus magic the group sells might also be making evil guys more powerful.
A clever paladin might work with the group, using the sale of evil stuff as a sting operation to find and investigate then capture/kill the buyers so that the group makes money and evil takes a boot to the head. Since the adventurers will then likely have a bigger cache of evil toys, they can then try to sell the items to someone else until people catch on. At that point, no one will be buying the items anyway, so he might as well break them. However, Clever doesn't come standard on pallies.
As far as spells go, any wizard who can cast Conjuration spells like Summon Monster, Planar Binding, or Gate can already cast [Evil] spells. If you can trust them to use those spells responsibly, then what business do you have burning up spellbooks because of a few evil spells? Additionally, the [Good] version of the spell is not always appropriate. If someone Gates in a Solar, and then it dies, well, oops, you just killed some high up Angel. When calling outsiders for very dangerous jobs, it might be better to go with the evil option so you don't kill some important agent of good. Besides, as a fringe benefit, Balors make great bombs.
EDIT: Oh yeah, I forgot to talk about the truly dangerous stuff. People don't use artifact type thingies; the artifacts use them. The Sword of Kas needs to be handled more carefully than nuclear waste. Anyone taking swings with is either making a brave sacrifice to beat some monster with Epic DR, or is a moron.