After the release of Wrath there was a big crash on the price of the Outland enchanting materials (Arcane Dust, Greater Planar Essence and Large Prismatic Shards). No one was interested in old enchants and there was a massive supply of them thanks to Death Knights.
Things seem to be heading in a completely opposite direction now. The materials are needed by enchanters leveling their profession and now is a very popular time for people to do this (Death Knights leveling up now or people taking on the profession because of its usefulness as raiding/arena starts in Wrath). Burning Crusade enchants are also popular at the moment because people do not want to waste money on expensive Northrend enchants while they are still gearing up in raids or arena, so they settle for the Outland ones.
On my realm Greater Planars have gone from 7g to about 17gold in the last few weeks. Arcane Dust has gone over the price they sold for during Burning Crusade. Large Prismatic shards are still worth very little because people still avoid the most expensive Outland enchants that required them.
If you do happen to have Outland gear you want to disenchant, here is a guide on if you should vendor or disenchant;
- Level 65-70 Blue quality or Level 70 Purple quality items; vendor these because you're more likely to get large prismatic shards that sell for a few gold on the auction house. Level 70 purples can vendor for more than 10 gold.
- Level 60-65 greens; These are more likely to disenchant to Arcane Dust so it depends on the vendor value. If they vendor for 2 gold, you're more likely to make a profit from the dust. If they vendor for 8g you might be better of just selling them.
- Level 65-70 greens; These are the opposite of the above. These have a higher chance of rewarding Planars, so unless they vendor for over 10g, you're better of disenchanting.
Of course there are add-ons that can help you determine this, but that's my tip of the day. This ignores listing the gear on the auction house. This should be avoided because there is very little demand and very high deposit costs. Most likely you'll waste a deposit for a week before selling, meaning it's hard to break even selling gear on the auction house.