Value is such as tough call. As others have mentioned, the community as a whole will speak up if you are getting hosed. But I like to look at it in terms of what is it worth to you when trading with another?
There may be items you find that are very rare, yet you have no use for it. And then you may find another who has medium rare items you would really like and can use your rare as currency. I recently gave up a few medium pieces to get a very rare piece that my wife wanted (soul cage), so I came out good. The other got rid of a spare and got pieces he was in search of in one fulfillment. Value is going to be determined by those who trade together at any given time frame.
And I would suggest keeping all gems. I literally have over 1,000 gems and over 500 of them are perfect gems. I run across trades and I need to make up a bit and offer Pgems in exchange. While I do not use gems in bulk right now, there are many who do and they will give you your building pieces (gear) for pgems; especially when they are extras to them.
Read the trading forum. You can get an idea of what some players consider valuable in trades.
Last but not least, do not be afraid to ask. I have received quality gear in trades with others just for asking to trade with what I have. While I don't carry the real high end stuff, I do have a good collection of medium gear that others need. I have taken gear off old characters to trade also since that gear right now is not being used, and I am in need of new gear for a new character build. Offer what you can, keep fingers crossed, and save all your uniques so you too will have a stash of gear for trade in the future.
Condor