If you know what item turns into what, what it's used for and so forth then you can put it up based on what you think it should be put up for.
This is a good point. Check out our Item Database
to get an idea what your item is used for.
Generally, if I have an item that nobody else is selling, I'll sell it for a bit more than what it would go for normally (but not by an absurd amount).
For example, let's look at a Tallow Candle
. Right now there are only two others listing it, and it's listed at 999,999,999 (aka, it may as well not even be there).
The Tallow Candle is used in a Lantern Shield
-- a fairly popular item in and of itself. The Lantern Shield is used to make a Bladed Shield
Now, I have a Tallow Candle +3. People want HQ gear. HQ gear need HQ ingredients. If someone wants an HQ Bladed Shield, they'll want an HQ Lantern Shield, which will in turn need HQ ingredients. So my list price of 10k for the +3 Tallow Candle is 1) pretty fair and 2) a good profit. Edited, Feb 5th 2011 8:45pm by Osarion