Sometimes I humor the idea of destashing some yarn. I pull out all of my yarn (it's really not that much, just a quarter closet full), I separate the bins, and decide where I am going to put the "destash pile".
First I start with my massive amount of sock yarn. I pull out the skeins that I definitely want to keep, put them back into the bin. Then I pull out the ones that I have ideas for, put them back into the bin. Then I pull out the ones that I am not sure of and put them aside for the destash pile.
I do the same for the rest of my yarn weights. These don't take as much time because I like to knit with heavier-than-sock weights. These are often tagged for sweaters and gifts.
I give my destash pile a once-over and pull out the balls/skeins that still speak to me...crazy colors - SOCKS for my daughter, too-similar color - SHAWL for a gift, too small yardage - DOLL CLOTHES for my niece, you get the idea.
I then take a final look at my destash pile and wonder if people will really pay $5 for even a lot for the yarn I first bought when I started knitting 3+ years ago...
And there you have it...the answer to my post title.