I haven't turned anything over in my collection ... yet. Finding it increasingly difficult though, especially when I see a card on the market from a tough set that I already have a sample of in my type set (like a YumYum).
Fact is that I only buy things I REALLY like, so the chances of me parting with anything are fat and slim. I'm resigned to the fact that I just buy what I like within my financial limits, and if I now have 4 Gypsy Queens, then I'll have to suffer with them ...
Yet, I can see a day where I would sell a part of my collection to fund a card I could not afford otherwise, like a Just So or 4-Base Hits. Maybe someday!