LessPop_MoreFizz's answer covers non-randomized commodities very well.
However, for randomized items, it is very difficult to compare most equipment types.
Generally, I estimate how much gold I earn in an hour at that level, and price the rares I sell with a buyout of roughly one hour's worth of gold accumulated at that level (what I would earn through normal quest progression). If the item has a stat that seems likely to make it particularly valuable (e.g. + to magic find, unusually high amounts of stats likely to be attractive to a class that would use it, + damage, etc.), I will bump up the price to 2 hours worth of gold accumulation.
However, weapons are a big exception. Since DPS is so important to every class, any weapon I find that has a high DPS gets a very different pricing strategy. I pick the class that I think would be most likely to use that weapon based on weapon type and secondary attributes (e.g. a dagger with high +int would cause me to consider it a WD weapon, an axe with high +str would make me look at barbarians, etc.), and then look for comparable rare weapons for that class in that level range. I then compare the stats to the other weapons with similar (+/- 3) DPS, determine if my stats are significantly better or worse than the competition, and then price either slightly below or slightly above (+/- 10%) the cheapest weapon in that DPS range, depending on whether I think the secondary stats are better.
If the weapon I'm selling is socketed, and the others in that DPS range are not, I'll mentally bump up the DPS several points since adding a ruby will add quite a bit of DPS, making it much more valuable.
This strategy gets me a steady stream of smaller amounts of gold from the items that are nice, but not fantastic, but also gets me large spikes of spending cash from when I find that really awesome weapon that just isn't quite right for me.
The most important consideration, though, is ALWAYS set a buy-out price. Few people will wait 3 days for an item, and the only thing you are likely to accomplish with a "highest-bid" auction is people trying to snipe it for cheap at the very end of the auction.
Edit: After almost a month, and after starting farming Inferno, both the market and my strategies have changed significantly. Here are some additional thoughts:
For high-end items, the strategies for pricing are rather different. To get the most for the items you find that are actually good, there are some specific things you need to pay attention to.
The first thing is to know what is actually good.
Some stats automatically add value to an item, but even then, the overall combination of stats are the most important. Still, any item with any of the following stats is automatically worth a second glance, to see if the item is worth something:
- Improved Attack Speed
- Faster Movement Speed
- Rings or amulets with a damage range (e.g. "12-30 damage")
- +to all resistances
- % life
- Life on Hit
- A primary stat (Str, Int, or Dex) > 200
- High DPS weapon (800+)
Other stats that add value if they are on an item that is already good are:
- +Crit Chance
- +to Crit Damage
- Magic Find
- Gold Find
- Bonus Damage vs. Elites
- specific resistances (physical resistance seems to be the most popular)
- Other "on hit" effects (stun/fear/knockback/immobilize etc.)
- Sockets (although these are more "mandatory" than "bonus" for chest and leg armor, in which case having only 2 sockets makes the item less desirable than if it had 3)
If you've got an item that has any of the above characteristics, evaluate the overall combination of stats.
This makes it important to know at least the basic strategies of each class, but as a general rule of thumb:
- +Vit pairs well with everything
- "Wasted" stats decrease the value of the item. If an item has +Int and +Dex, one of those will be seen as relatively useless by whoever uses it. There are a few oddball exceptions (e.g. Witch Doctors looking for +Str and +Int because they want to try boosting their pets), but the oddball exceptions don't net you a lot of cash.
- Great stats aren't that great if a class can't find at least 100 points in their primary. Gloves with +ias are always good, but if, as a Wizard, I see +ias, + x from health globes, Health Regen, and Resist All, I'd rather spend my money on something with +ias and +130 Int.
- The best money is for the items that have 200+ in a primary stat, plus other great stats.
- Get a feel for the class-specific desires. Witch Doctors tend to look for Mana Regen, or Mojos with a high damage range (anything with a top end over 200 is decent; anything with a top end over 300 is good), whereas DH quivers don't seem to sell well in general (at least I've had no luck with them).
As far as pricing, I'd like to add one basic rule that covers everything you put up for auction:
Always, ALWAYS increase the starting bid! The system defaults the starting bid of an item to the amount you'd get from vendoring it. However, you pay a "transaction fee" if your item sells, meaning you get only 85% of what the item actually sells for (on the gold auction house). If you leave the default starting bid, and someone bids the minimum amount, and no one else bids, you will lose money.
For good items (based on the rough criteria I described above), a few useful tips:
- Check the prices of similar items. Remember that just because similar items to yours are listed with really high prices doesn't necessarily mean anyone is paying those prices. If the competition seems too high, chances are that they are (particularly if there is only a couple of dozen or less items available).
- The lower your price is compared to the competition, the less the chance of you having to wait out the entire auction duration. You need to evaluate how important a quick sale vs. getting as much as you can for the item is, though (and don't forget that AH cut!).
- Cheap trick, but effective: try to knock off a digit if you can do so without decreasing the value too much. Changing the price from 1,000,000 to 999,000 may seem like a cheesey gimick, but it works. When people are scanning the prices, yours will jump out from the rest, particularly if you really are cheaper than the rest. When items of the same quality range run 1,100,000 - 2,000,000, and you post something for 1,000,000, it looks cheap, which could entice a sale, or could wind up with potential customers saving up for the "ideal" item. When you post it for 999,000, though, it looks like a bargain, and someone will snatch it up.