Value vs price is a tough one. I know I have struggled when I put stuff up on the EE. I almost always sell my items within two BTT and have had to pull an ad only a handful of times to re-list something because I priced it incorrectly. The general approach I take is:
1.) If the used price that I list the item for is tolerable for me to absorb, I list the gun and get on with it.
2.) If the used price is not tolerable but I am trying to get something else that I need/want, I try to trade into the item I am looking at. In general, you will get a more reasonable trade after you account for shipping IMO then taking a loss selling a gun and then buying again. I find it hurts less financially but the only negative is that you need to be willing to accept an item that is reasonably close to what you want. If you are one of those cats that must have the exact model that you want (which is totally cool) then trading becomes almost impossible. That being said I have scored a few times on the exact item I wanted on the trade but it is not necessarily going to happen unless you are really patient.
Just how I get deals done on the EE.