The issue here is not whether something will sell, (eventually, many things will), but rather is it worthwhile to keep your stash/bags full of items with mediocre stats, some of which require 2 to 3 AH listings to move?
Right now I believe it's more cost effective to vendor rares, but that may change in patch 1.0.3, in which Blizz has announced that they are "...looking to adjust the Blacksmith costs for training (gold and pages) and crafting from levels 1-59..." In this case I hope "adjusting" means "reducing."
My method for rare item triage has lots of room for improvement, but this is what I've been doing when I encounter a rare:
- Is it an upgrade for me? If yes, hooray! If not, then,
- Is it an upgrade for someone in my party? If yes, give it away. If not,
- Do I have an alt that can use it (sooner than later)? If not,
- Are the stats amazing? Does the item make you think "boy, I wish I had a [insert class here]"? If so, then list it on the AH for a lot. If not,
- Does the item have stats that are decent, AND the item has +magic find and/or +gold find? If so, list it on the AH. (I admit I have spent 15000-25000 gold on something 10 levels too low for my "gold accumulation" set.) Finally,
- If #1-5 don't apply, then vendor it.
All that said, I have trouble following my own advice sometimes. I'm good about vendoring rares with truly horrific stats. It's the items right in the middle I have trouble parting with.