This has been an ongoing issue for me since I started. I'm the type of person who will read and research something endlessly, but this seems to be one of the few areas where I was just stumped... because there is NO easy answer.
I've been playing since October, and here is the very general criteria I've started to use "on the fly" - meaning, the criteria I use when I'm trying to decide quickly what to sell and what to keep and what to look into further...
1. A blue or purple item which has a requirement of 8 or 9, and does max damage, I will hold onto until I have more time to look into whether or not people are really into it.
This thread is a quick reference of items/requirements/stats compiled in one place. I
now know the max damage for each class of weapon off the top of my head, but early on, I'd just look on that list for like, axes - max damage of 6-28.
2. With trophy drops (all the little stacking bits like kournan pendants, icy lodestones, krait skins, plague idols, etc.) I've started looking each of them up on the Wiki. I keep ones that either Nick has not yet collected, or that are sought by Wintersday/Halloween/Canthan Chef collectors, and I store them in storage characters. I ONLY started doing this recently, once I decided I was truly committed to this game and it was worth (to me) investing in extra characters.
3. Gold weapons I will almost always investigate pretty thoroughly. I make use of the
GWEstimator to see if people are buying/selling them and for what price. I've gotten better at identifying immediately if something is merch food (high requirement, less-than-max inherent stats, etc) so I don't have to research quite so much... but early on, I was honestly looking up everything I found.
4. I will salvage off max
modifiers or
inscriptions that are
worth more than a couple of platinum.
5. I have a list I wrote out of Runes & Insignia that are worth more than just the standard 25gold. I basically went to the trader, looked at the selling prices of all the Runes/Insignia that were going for more than like 150g, and wrote them down. I can quickly scan that list and know that a Major Fire rune is pretty worthless, but a Major Vigor is absolutely worth salvaging. The trader will give me a lot for a Warrior's Sentinel, but next to nothing for a Sentry.
6. I'm always looking for weapons/mods/runes & insignia I want for my heroes.
7. Anything Green I
price check here.
8. I periodically scan the
WTB Forum here, to see what sort of things people are collecting.
9. I pay really close attention to "named" things... names I see frequently coming up in the trade chat in Kamadan: Celestial, Plagueborn, Voltaic, etc.
10. When in doubt, I ask here on Guru. I don't really trust the price checks I see given in cities over the chat... but here, the community sort of keeps people honest, even if as individuals a few might be inclined to try and cheat. I also make heavy use of the Search function here... "Diamond Aegis" in quotes, "Ruby Maul" in quotes, etc. Helps me find what others have price checked, or have bought/sold them for in the past.
When I first started, I REALLY wanted there to just be a simple list somewhere I could consult... and I couldn't really understand why it didn't exist. I "get it" now. It would be impossible to create or maintain. The value of things is something you learn over time, sometimes by making mistakes, but usually by devoting the time to researching the items. I enjoy that part of things, so I have no problem spending time on it.