When I was thinning the herd several years back, one criterion was, "Do I remember who this is?" If I couldn't remember the character's name, he was a prime candidate for selling. I figured if I couldn't remember who the figure represented, I was unlikely to need it for a display or story. I tried to be somewhat conservative in how much I sold, though - if I regretted a sale, it would be hard to get the figure back. Thus far I haven't missed any of them.