For camera accessories, usually I don't mind. For lenses, hmmm... I have tried many times in the past to let go 1 or 2 but somehow I find I have a use for them, even though I don't use them often. You can say it's a sentimental feeling.
For eg, I have a $400 lens, and if I sell it for about $300, I'm making a loss of $100. (The lens is in tip-top condition btw.) And even then, $300 is not a lot to be gained. Seriously, unless you own a basic camera gear, otherwise, $300 is peanuts in a hobby such as photography. The only reason I can think of for selling it away is to make more space in my cabinet. I use that lens very minimally, but on those occassions that I do, this lens never fails me and I feel more expressive.
Now I tell myself, if I really sell it away, it'll probably be the day when I give up photography or want to keep my camera gear to very few items only.