I'm newish at the latest incarnation of the game and have only been playing 2-3 months now. What's starting to tick me off is all the attitude I keep running into with trades. In one game I'll trade a desirable unique for another while in others I run into some snooty l33t speaking git telling me something is worth at least this or that absurdly high rune and that my offer is a total joke. Sure Death's Web is valuable but why should I have to locate a rune that is ten times harder to find than it?
[color=#33ff33] People on public and not from the forums tend to use just about anything for a value, usually high runes, Perfect Gems to some extent, Soj's and duped items (290's 3/20/20's etc.) though the use of duped items in any trade on the forums is disallowed.
As far as the trade forums here, the trade forums have a trade value list that is accepted by most members and updated here and there. Also note the dupes list and do not trade lists, items appearing on those lists are not allowed to be traded off of the forums.
Of course, remember that most sellers will want more than the market price and most buyers will want to pay less. Also, lots of people who find only 1 or 2 decent items will consider them absolutely godly and will want 100 zod runes for them.
I've decided that if something I find is worth, say, 7 um runes, it's not worth my time to ask for 7 ums; I'll be in trading channels and forums for days. So I just ask for 6 ums, unload the item, and get on with playing rather than wasting all my time trading. Of course, that's kinda the same mentality that would lead one to buy items off of ebay (that your time saved is worth more than the money/items spent).